Other than feeling super sexy on, and showing all of your manly delights, why would someone wear compression shorts/tights...To keep your manhood tame?
Depends on the type:
Recovery for .... Recovery
Baselayer type: leg day. I can't deal with plastic rubbing against my legs and causing burns and rashes. Also when wearing tight trousers. Because they are thinner than briefs, and squashes things, it makes it easier to get into tight things like sports kit or skinny jeans
Thermal compression: it's cold