You don’t need one, exactly. But even if you don’t wear your jeans baggy intentionally, they usually stretch out a bit as you wear them between washings, and the belt helps keep them in place. It may not seem like much if they only slide down your waist a tiny amount. But if you do tuck your shirt in, it may make a difference in whether the shirt looks like it fits or not. The belt also helps to keep them from gapping out, especially in the back when you sit down.
That said, I do omit the belt anyway sometimes, because I like the way it looks without them.
To answer the OP: Tradition. Levi’s, the oldest denim pants (dungarees) producer in the US, has always put the label in that location. Levi’s jeans also use to include rivets at all of the stress points. The rivets have slowly seen a disappearance. Most jeans now have extra stitching on these points. In the early 1900’s cowhands requested the rivet at the crotch area be removed because it would heat up when they were crouched in front of the campfires in the evening, causing considerable distress. In the 1950’s schools all across the country requested that the rivets on the back pockets be removed, as they were causing desk chairs to become more quickly scratched and damaged.