If’n you got crabs, it is! Cover your junk!
As long as your pants are up, it’s OK with me.
Most people wear them because pants can get rough on sensitive skin, and skirts can get awfully breezy. Underwear will provide both protection and some warmth.
Some boys need underwear to support themselves too. I’m 13 and I get irritated if I don’t have briefs on. Sometimes it’s annoying even when I’m wearing them, especially when you’re running in gym class.