I'd say it's a hobby. Bullets are expensive in quantity. A full automatic can chew through $100 worth in just a minute. There are a few gun ranges near me that keep them available to rent. You can relive your A-Team / CoD dreams for a few hundred dollars.

Outside of the military they have no utility whatsoever.

Full auto on an M16/AR15 will empty a 30 round magazine in a touch over 3 seconds. A few ranges near me will rent you a full auto AR but you have buy the ammunition from them.

The primary benefit of high rate of fire is hitting a single moving target. Machine guns used to be particularly common in air to air combat. When you are a school shooter or bank robber you actually don't want to waste your ammo like that. The downsides of reloading are by far stacked against you. A soldier can just go back to base and reload his guns but a robber whose magazines have run out will just see his targets flee and his only opportunity slip away.

There are a few subgun matches, but IMHO it's mostly just people wanting anything the government says they can't have.