How effective are they in influencing the people who watch them? Do people really see an advert and think ‘hmm I like that fridge so I’ll buy it even though I already have a fine one now.’ Or is it a more sort of insidious process, where you see an advert and it sticks in your memory until the time comes and you need to buy a new product so you choose the brand that you saw on the TV. I never really believed they had an influence on peoples behavior in regards to what they purchase but if companies are spending millions every year on their adverts then they must have their uses.
I’m pretty sure it’s the latter - it’s when whatever you’ve got gives up the ghost and you need to buy a replacement, you’ve got their brand in your head.
Old deodorant is used up? Thinking about how it didn’t work so great? “Hey, I remember those funny Old Spice ads, maybe I’ll try that. It seems popular.”
Old CRT tv gave up? “Well, Samsung has a lot of commercials out now about the color depth in their new flat screens. I’ve heard they’re good too so maybe I’ll ask about them at the store.”
So on and so forth.
It’s mostly about presence and the ability to come first to a user’s mind when they start having dissatisfaction with their current product. “Allstate didn’t handle that claim too well, and Geico claims to have better rates in their ads so…”
This is the same idea behind branding and logos. Keep a consistent face where people can see it and when they say “hey I need a new X” your company may spring to their mind. If you’re not consistently presented to the consumer you get things like, “there were those new dryer ball things…what were they called?..I can’t remember…” and then you’ve lost the sale. With a good logo and some ads they might at least remember something, if not the whole name and jingle. And of course, ads give you the opportunity to make your product look cool, reliable, fashionable, etc. to pander to a certain demographic and stick in their heads better. That’s why you get a lot of denture cream commercials during the time slot for The Price is Right - that’s the largest demographic watching, and the demographic most likely to buy your product when their last tube runs out.
The more you hear about a product/company - the more “relevant” and “trustworthy” is appears.
So, would you rather spend your money on something you know nothing about, or on something you hear about all the time?
I will always buy Jif peanut butter because “Choosy moms choose Jif.” I used to hear that commercial many years before my children were even a sparkle in my eye.
I always find it amusing to see a Boeing ad. I’m not currently in the market for a 737, but who knows.
Let’s just say that you’re not the primary target for those ads. 
(I used to work in advertising, and Boeing was a client.)
Relatively few ads have the immediate goal of “go buy this NOW!” More often, they’re intended to do one of a few things:
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Build your awareness of what the product / brand is about. If you hear “Geico”, and think “I can save money in just 15 minutes”, chalk that up to seeing thousands of Geico ads which told you that.
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Keep the brand at or near the top of your mind, so that when you do find yourself in the market to make a purchase, you’ll think of that brand.
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Let you know about a new feature / benefit / flavor of that product. “Hey, Burger King has a new sandwich…maybe I’ll have that.”
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Build an association in your mind between the brand and something else (a place, an activity, an emotion, etc.) Corona beer tends to get associated with a laid-back, beach lifestyle, thanks largely to years of ads based on this idea (not to mention years of sponsoring Jimmy Buffet).
TL;DR: in answer to the OP – yes, they do work, which is why they spend the money, but the definition of “work” may be broader than you think.
Most of the responses have it right - advertising is very little about “buy this now” and a lot about embedding brand, product and feature points in peoples’ heads for later influence. (Smugly saying that advertising doesn’t influence you is akin to saying you don’t absorb any O2 when you breathe.)
What’s either frustrating or amusing, depending on your perspective, is that a lot of businesses don’t understand how advertising works, either, and do their marketing on the basis of turning it on when times are good and turning it off when times are slow… and expecting solo or limited-reach ads to do much of anything.
“A man who stops advertising to save money is like a man who stops a clock to save time.” - Henry Ford
Does anyone really believe that Coke and Pepsi taste objectively better than store brand cola? Our preference for brand names is often based on subjective associations created by advertising.
Well, that’s simplistic, at least for food products. Most big brands do sell something other than the color of the label.
What Coke, Pepsi, McDonalds and the rest offer is *consistency *- a Coke in LA is going to taste the same as a Coke in NYC or Ohio or Texas or East Jeebus, Kaintuck. More generic or regional brands will vary; a regional or store brand cola may definitely taste like a cola, but it may have a variant flavor you don’t care for.
Non-food products, not so much.