Purpose of tank tops as undershirts (men)?

What function do they serve and what do they offer that other undershirts don't?

They're cooler in summer, don't hang out the arms of a short sleeve shirt like a typical undershirt might and the "wife beater" is an integral part of the basic redneck wardrobe. :-)

Not going to be detailed about this, but if you are in the woods a long way from home tank tops can be a use a few different ways of survival.
Just think about it.

My husband is really muscular. I think the purpose of his is to give me an incentive to get our son to bed on time. ;-)

And the lint free ones make really good cloths for furniture refinishing (staining furniture, wiping away sanding dust, polishing, waxing, etc.). Works well for me. I've also washed brand new ones and used them like cheesecloth to make tofu in a pinch.