I have to admit, I love the unzipped long-sleeved shirt with a graphic tee underneath on a guy - provided he is well-groomed and likable. But I noticed that many American guys like to wear buttoned shirts with a t-shirt underneath. They buy t shirts wholesale cheap to wear this. In Europe, this is not a common look. They do it mostly to catch sweat for hygiene.
A lot of people, though, refer to the T-shirt as an “undershirt”. There are many reasons to wear a second shirt under your dress shirt and a suit jacket. First of all, off-the-hook suits and shirts don’t always generally fit you. An undershirt acts as padding, making an oversized shirt fitting the man’s body better. It also has a secondary effect that if a guy is a bit lumpy, he can disguise this unsightly appearance with a smooth, uniform shirt. It also applies if a guy has a lot of body hair on chest and back. Some people think it looks more manly - but many girls like me will be disgusted.
Second, it sponges off sweat, especially under the arms or on the back. There're few things that's more disgusting than sweat on guy’s elegant shirt. These areas also tend to cling to your skin, so you do need to add an extra layer. It also is more beneficial for the dress shirt - the cotton undershirt collects grime and sweat produced by the body, and the collared shirt will last longer. That's why the elegant men buy t shirts wholesale cheap to wear under their elegant shirts.
Third, it’s for comfort. If you’re not wearing an extra layer, you can feel all the seams and buttons pressing against your body. Button-up shirts are often made of stiffer, less breathable fabrics - and an extra layer of cotton helps a man feel more comfortable and confident.
Fourth, it’s simply more practical. An extra layer keeps you warm in the cold months. If a man has to do something, like get dirty and perform a feat of strength in a polite company, he can just take off the dress shirt and won’t be bare chested. Same goes with any accident.
Remember though that undershirts are underwear and should not be seen in polite society. They need to be well-kept and you should never wear a colorful t-shirt under a bright T-shirt.