Ok, fight my ignorance.
I used to think that a snorkel was used to swim just below the surface and continually breathe given that the top of the tube is stuck into the air.
But I see pictures of people “snorkeling” several feet or more underwater. What’s the deal?
You hold your breath, let the snorkel fill with water, and swim downward – to a depth that’s still comfortable on your ears (usually 10-20ft). It lasts as long as you can hold your breath.
When you come up, the snorkel has a valve on it that lets you clear out the water easily.
You get air on the surface, swim down for whatever reason, then resurface. You violently blow the water out of the tube and then, just breathe normally.
Just because you’re snorkeling doesn’t mean you’re required to stay on the surface. If you see something underwater you want to see closer, you’re allowed to. You don’t need to continuously breathe.
Honestly, it’s a pretty strange question. Unless you’re talking about a Snorkel Hookah or similar device. Those are a bit different in that they allow breathing at deeper depths (max of about 40’) than snorkels via surface floating compressor. But several feet or even up to 15-20’ down is possible on a short dive from surface without weights or external air source. If you want to see some seriously crazy shit, google free diving.
I thought it was a weird question too. But I was a child in San Diego and swimming in the Pacific Ocean from about the time I learned how to swim. Using a snorkel was just part of growing up. But not everyone lives near the ocean. Some places, there are other activities to do in the nearby bodies of water, and snorkels might not be so common. I used snorkels in swimming pools, but not everyone does. If someone doesn’t grow up with snorkels, it’s an understandable question.
I haven’t snorkeled for a while, but pressure on your ears should not be a problem as it is easy to equalize it. That said you don’t want to go too deep anyway as you necessarily need to come up quickly when snorkeling.
Snorkel = Skin Diving
Air Tank = Scuba Diving
And snorkles are used a lot for ‘scouting’ out things of interest, like reef structures/corals, etc) to swim down to - lots more looking time -v- swim down, look around, then resurface. Easier to just amble about without taking eyes off ‘ground’ under you. I used a snorkle a lot when spearfishing some Texas lakes Lake Travis near Austin especially - could see huge catfish/bass hanging out and easy to go get for keeps without much effort at all. Kinda felt like cheating, LOL.
This was the old snorkel design - in the last few years the std snorkel/mask packaged snorkel has a nifty valve design that closes when you submerge, and then opens when you re-surface - no need to clear it of water. Otherwise, I agree with all the comments about diving down - if you want to get a closer look at a fish, sea turtle, coral, or whatever.
*That said you don’t want to go too deep anyway as you necessarily need to come up quickly when snorkeling. *
I’m afraid this just isn’t true. “Snorkeling” is something tourists do on vacation - but the serious sport of diving without a tank (just by holding your breath) is called “freediving” and the current depth record is over 400 ft.
Snorkeling is widely used by non-tourists. I’ve used snorkle for scouting out fish for spearing, ‘noodling’ catfish’, for simple looking around neat rocky formations underwater - without needing to lift head constantly. Another great example is how the Gator Boys (TV series about ‘rescuing’ out-of-element gators - note snorkel on person in first pic of link :-). Those guys use snorkle to swim around known area of gator’s location (lake/pond, etc), then swimmer goes down deeper to try and snag gator for capture. A daring task, but snorkel gives serious advantage -v- no snorkel, for sure, in such situations. I also, during my days of whitewater rafting/‘yakking on Snake/Salmon rivers in Idaho to always take along mask/snorkel as it would be of tremendous help in retrieving a person and/or gear that became stuck underwater in big currents, etc, should such event happen. I also once used snorkel/mask to free a wrapped-in-debris ~10’ sturgeon I had hooked a few hours earlier not too far downriver from Hagerman, ID (reeling in big sturg’s that size in heavy current can take a loooong time). My buddy held onto pole and I was able to not have to raise my head to breathe as I cut through/removed the branches and barely submerged debris/trash sturgeon had swum through near river’s bank and finally freed the protected species so it could live/breed another day. Been dang near impossible to have done so without being able to not take my eyes away from line/fish while doing the labor. A useful tool, indeed 
Snorkeling is a VERY useful tool for a variety of objectives other than ‘tourist-based activities’. Freediving is not about seeing what is below you - it has to be known before diving down that its deep enough for expected challenges of said freediving. Its not like freedivers swim around known-shallow waters to go deep, ya know?
Snorkeling allows you to cruise the surface with eyes (mask) in the water to look at things. Then, if you want a closer look, you can dive down for a closer look - easier head start, easier to find where you want to go, etc. As mentioned, when you surface, blow out snorkel, resume cruising the surface. Better than no snorkel, less cumbersome than full tank gear.
My comment was about the idea that there is a limit to how deep go you “should” go when holding your breath, because you necessarily need to come up quickly when snorkeling.
The admonition for divers to surface slowly is for compressed air divers who could possibly get an air embolism as the compressed air in their lungs expands as they surface. There is no such issue with breath hold diving as evidenced by the fact that people can, and do, go very deep with breath hold diving.
The issue of “the bends” (decompression sickness) is a different matter and has only ever been seen in breath holding divers who have made a series of deep dives with little recovery time at the surface - definitely not a problem for anyone but the most experienced deep divers.
As an aside, I have had many more sunburns on my back (torso/legs/neck) than frontal areas when fishing/swimming than anywhere else due to my back usually being exposed so much more than other body parts, LOL. Usually was teased whenever I took off shirt as tanning level was much ‘deeper’ than on my front side. I used to spearfish lakes around Austin a LOT back in the days before LCRA park became Mansfield Dam Park, a tourist attraction/improved for better public use, etc. Right below Mansfield dam was excellent place for the HUGE catfish, too. That was during time when highway still went across actual dam itself instead of the off-dam highway present nowadays.
And,** Ornery Bob**, I hope I did not come across as snarky to you. Not meant, anyways. Freediving is totally different approach to purpose of being ‘in-water’ than snorkelers, of course. Freedivers amaze me at their ability to disappear so deep unaided!
I once CT’d a Doc I worked for (chest problems) and usual approach is to have patient take deep breath, do a few slices, then have patient rebreathe and do more slicing, rinse/repeat. This Doc, who once floated on puncture-proofed inner-tube entire length of Snake River as enviro-protest attention-getter against dams, was able to hold one breath for the 4 minutes exam took (older model non-spiral unit, MUCH faster nowadays). He was an avid freediver in deep/cold/clear lakes around the mountainous area where we were, fwiw. I and other Tech were getting worried about brain O2 starvation, etc, around 3 minutes into exam, but Doc kept his thumb-up sign constant for ‘positive alertness’ we had agreed upon that he was fine prior to starting exam we did not think he could finish in any way. Both us Techs gave him a huge bow at waist for such endurance, LOL. Never again have I had anyone come close to such hold-breath time for whatever reason. Amazing what conditioning can do regarding physiology and such.
'Cause that would be snarkeling.
Bitchin’ dry snorkel. You really need to get your hands on one to appreciate how they work, but it’s pretty self explanatory. Snorkels with a purge valve on the bottom are SO much nicer than the old fashioned radiator hose rigs from when you were a kid. The purge valve allows close to half of the snorkel’s volume to drain out the bottom when you surface, making it much easier to blow out the rest. With the old styles, you had to blow out the whole column of water–something approaching a pound of water–which is a pain in the ass when you’re already about to gasp for a fresh lung of air. Also, the purge valve allows you to have a bigger diameter tube so you can breathe a lot easier.
My snorkel is of that type. (I haven’t been diving in years. Might be a Scubapro.) But it doesn’t have the thing on top; just the valve on the bottom.
Historical note: I thought the days of breathing in lungs full of water were over when, as a child, I got a snorkel that closed up automatically. This was a toy snorkel that was like an elongated ‘S’ with a mouthpiece on the bottom and a ‘cage’ on the top. The ‘cage’ had a ball within. The idea was that if you submerged, the ball would plug the top of the snorkel and keep the water out. When it worked, you suddenly couldn’t breathe. When it didn’t, you still got a lungful of water.
We do this here in S. Florida quite often. There is a shallow reef system just off the coast - it’s an easy swim in some areas. So we put our snorkel gear on, swim out for 10 minutes, and then can check out the reef. It’s in 10-20 feet of water. We’ll take a deep breath and dive down to check out the holes, take pictures of the cute little sharks, etc.
I reread the OP. I think Bricker should be satisfied with everyone’s explanations as to why, so far. IMHO, the reason you see so many pictures of people snorkeling underwater is that it probably makes a better photo than trying to capture someone floating at the water’s surface, half-in/half-out.
As a child, I spend almost a year living on Kwajalein in the Marshall Islands. I have no idea how many hours I spent snorkeling in the lagoon, with a mouthpiece that was nothing more than a bent tube, no valves, nothing to stop water from entering if you dove down for a better look at a shell or whatever. A good strong exhale when you surfaced usually cleared the snorkel of water though, if not, you just took it off and shook it.
The water was so clear you could see the bottom 15 to 20 feet below while snorkeling.