Well, until Sunday night, they were used for the recreational simulation of full-auto shooting, by those who can't afford $10k+ for a pre-1986 full-auto firearm. (This based on my inability to find any reports of any bump fire stock being used in any crime prior to 01 Oct 2017.)

And even then, they're only used by people who can afford to blow off $9 worth of ammunition in about 4 seconds.

I've used one - once. First thought was "WOW! That was fun!". Second thought was "wow - that was $90 in 2 minutes - I'm done".