Do those devices have any legitimate use at all?

I think smaller ones might be useful for network quality testing and mapping. I think carriers drive around with boxes in vehicles to test their own networks reliability and map their competitors.

They have lots of illegitimate use that isn't about crashing the cell network like sending out spam https://www.cyberdaily.au/security/9949-sydney-man-arrested-... or allowing people to use in-network free call allowance to instead make voip international calls

They are mostly used for legal purposes, even if they might violate contracts with telcos.