Men wear underwear mainly for the extra comfort that it provides and to protect their pants from stains. Since one of the most common style of pants is jeans, which are made of a relatively rough fabric, underwear helps to keep the fabric from rubbing and scratching a man's legs.
Almost all men's pants have a zipper at the crotch. This zipper can scratch and cut at a man's private parts and can be very cold during winter weather. Underwear also keep the pants cleaner by keeping an extra layer between potential bodily fluids and the front and seat of the trousers. In the event of a spill of food or drink, underwear can help keep the liquid from soaking onto a man's legs.