Depending on your concern for modesty, it can also smooth out faint, uh… mushroom-like outlines. It also can keep you in check during those “spontaneous” moments.
It helps to prevent drip spots.
If it fits tight enough, it can keep your equipment out in front where it belongs instead of preparing you to audition for the The Crying Game: The Musical.
Neither here nor there, but in old west times, cowboys used to wear one pair of long underwear continuously for the entire winter. They’d cut it off their bodies in the spring.