To avoid cold. I do it myself. Moreover, this is what my friend Evgenich does, and indeed all his friends. In cold and windy weather, it warms up very much and does not allow you to blow through. :)
What T-shirt are you talking about? There is one). Long Sleeve T-shirt).
When a man (another other creature) sweats, the shirt does not get wet and the smell of sweat remains on the shirt. The same situation is with the shirts under which men wear T-shirts