A-Shirts Underneath: What's the Point?

Here's something that's always puzzled me: Why do men wear those ribbed tanks (technically called "A-shirts" but commonly known as "wife-beaters") under nice -- I mean well-made, expensive -- shirts?

If you don't like undershirts at all, that I can understand; in fact, on one level, it's immanently sensible to avoid wearing an undershirt if you prefer fine garments because it allows you to feel the exquisite material against your body all day.

But the reason the rest of us put on an undershirt -- that is, a tee -- with such clothing in particular is that we want to prevent our underarm perspiration from rendering it impossibly rank (you can get several days' use per washing with an under-tee) and, worse, ruining it with permanent discoloration. An A-shirt may keep the torso in general dry, but it doesn't address the majority of the problem, namely, pit sweat.

So why wear one, at least when the clothes are nice? Why not wear a tee to save the shirt or nothing at all to feel the fabric? Is it because A-shirts hug the body, making it feel sleek and taut? Then why not use them for other purposes, like pajamas? (This is what I do.) Or is it because it feels cool to look like a rapper (since most guys who wear these things for undershirts unfasten a button or two to let everyone see what they've got underneath)? Really, I could use a hand with this one.