You guys hygiene
undershirts/beaters are used to help prevent chest/back sweat from touching your shirts
"The basic purpose of an undershirt is to absorb your sweat. It’s there to provide a defensive layer between your body and your more expensive clothing.
A good undershirt can also provide insulation when needed, and some are worn to “compress” the figure in a slimming attempt. But for the most part it’s a sweat rag. Don’t shy away from thinking of it as such.
The easiest way to explain undershirts is to say that it’s pretty much exactly like underwear for your upper body. Some guys go without it entirely, and lots of guys have their own preferred style, but for most of us it’s a basic necessity that keeps the rest of our clothes from getting gross.
The reason we have so many different styles is that climates and the clothing worn in them can vary widely. A thick, knit undershirt with long sleeves and a high collar makes sense in a snowy winter, but the same wearer might want a short-sleeved variety with a deep V-neck or scoop neck in the summer, so that he can wear short sleeved shirts with the top couple buttons undone.
Add in the really high-tech ones — athletic gear and compression shirts, mostly — and you’ve got a wide range of products. Some are as plain and utilitarian as you can get; the upper-body equivalents of plain white cotton briefs. Others are stylishly designed for visible wear, especially in the athletic sector.
But at the end of the day, they’re all there for the same reason: your body eventually makes the clothes it’s touching pretty gross, and an undershirt is a lot cheaper to wash and eventually replace than your nice clothes."
Undershirts: The Man's Guide to Their History, Styles, and Which To Wear | The Art of Manliness
