To me, the main reason to wear a t-shirt is protect your dress shirt from perspiration. By wearing a t-shirt under my dress shirt, I can wear each dress shirt several times before taking it to the cleaners.
I wear v-neck or crew neck t-shirts, and I have no idea why anyone wears tank top t-shirts. Since a tank top t-shirt doesn’t cover your underarms, how can it protect your dress shirts from perspiration? And if it doesn’t protect your dress shirts from perspiration, what is the point of wearing one?
Not exactly an earthshaking question, I know, but it has been bothering me for a while.
Constantine
Maybe because they don’t want chest hair or nipples showing through their dress shirts?
Not really for dressy wear, but they are ideal for monster truck rallies and those intimate little gatherings where you sit in the abandoned sofa out in your front yard and yell for yer woman to bring ya another beer.
They’re also known as “wife beaters,” btw.
Warmth.
Haj, who has never worn one.
‘athletic undershirts’, as they are formally called, accentuate a man’s beer-gut.
naturally, chicks love this- hence the popularity among a certain segment of the population, which also gives this charming garment its popular name- the wifebeater.
I’m not sure why, but I find an athletic undershirt, when worn under a dress shirt of a svelte man, quite sexy.
I love wife beaters on the right guy. My husband isn’t the right guy.
When I wear them at work it is generally for warmth, particularily when the landlord decides to turn down the heater.
At home, it’s to catch my GF’s eye…which sometimes leads to things getting a bit warmer, as well.
People wear them so they can appear on COPS. 
I always think they make the shirt look “whiter.” And yes, I agree with JuanitaTech, I dig 'em under dress shirts. Just makes a man look more “finished.” T-shirts with sleeves don’t have the same effect.
Don’t dig 'em at all by themselves; they scream for saggy pants, large gold emblems on chains, and pointlessly expensive athletic shoes…unless they are paired with a cute pair of boxer briefs, and the guy’s got tight arms and a good ass.
Did I just say too much?
This is why I wear one. I have a fairly sheer white dress shirt, and didn’t want to show through, so I picked up a few tank top under shirts. The reason I didn’t go with a full regular style t-shirt is because I hate the extra fabric buncing up on my arms.
Well if you work out consistently and watch your diet, a wifebeater t-shirt can be incredibly sexy.
for the record, my dad is very fond of his wifebeaters. though, my dad was also a wifebeater himself. 
i could not deal without the sleeves in my undershirt…it keeps sweat from soaking through my armpits, as well as dripping down my sides. eww.
Hanes makes these for women. My partner and I both wear them. Sometimes I wear them instead of a bra (I can still do that for now) - and sometimes I like them because after wearing a suit all day at work when I come home I just get undressed and throw on my lounge pants and have that wifebeater on - In other words I don’t have to go to the trouble of dirtying up clothes for a few hours after work.
And, put a chick in one of them with a nice tan, smooth silky skin, and cut-off shorts… VERY sexy =)
How common is this practice of wearing an undershirt beneath a dress shirt? I’ve never noticed anyone in my office wearing two shirts (but of course, it’s fairly temperate on my side of Australia, even in winter). I’ve noticed old men (my father, for one :)) wearing sleeveless shirts (“singlets”) under business shirts, but no one under the age of 50.
Would you wear an undershirt beneath a white dress shirts? Woudln’t it “show through”?
hyperjes would agree. I never wore 'em before I dated her, now I wear them semi-regularly.
Narrad, a well-dressed gentlemen never wears a dress shirt sans an undershirt of some sort.
I hardly ever wear the tank top my wife got me. But when I do wear it, I do so for the following reasons:
a) There are a lot of places where being shirtless is inappropriate.
b) It’s slightly cooler than a regular T-shirt (average temp here: 80 F)
c) My wife likes muscular shoulders.
Oh, and I suppose d) so I don’t have a complete farmer tan. Of course, it isn’t an undershirt-style shirt, it’s reddish and has a big toothy yellow cartoon fish on it.
If I’m actually wearing an undershirt (I rarely have cause to wear formal shirts around here) I can’t stand the sleeves of the undershirt flopping about. I’d far prefer the sleeveless sort.