A jock is a bra for the balls. When I am running and jumping I don't want the jewels flipping and flopping around. When I was in high school, a long time ago, there was jocks, briefs and boxers and they made us all use a jock in PE. The point, I think, was to teach us all about this useful underwear option that many if us would have overlooked.
After high school I played rugby for years wearing briefs (Ha, ha. Yes I had other clothes on.) I should have used a cup but didn't like them and I felt the briefs gave prevented flopping around just fine. If I was a boxer guy I might have gone with a jock.
These days there are bike shorts. I think of men's bike shorts as a job-bra for the testes.