U.S. appeals court blocks ban on rapid-fire 'bump stocks'

Why? because it's fun! There was never a "need" for a bump stock. But back in the day of cheap ammo, they were a legal way to have the fun demonstrated by the popularity of the Knob Creek Machine Gun Shoot for 50 years. Sadly, I never got to attend before it ended after October 2021.
The bump stock evolved as a way to have the same sort of fun after the Federal government infringed the right to keep and bear arms with the 1986 ban on purchase of newly manufactured automatic weapons. That capped the number available to civilians to those already registered at the time the law went into effect, but it also increased the demand on the resource that would only become smaller over time. That drove the price of those firearms to several times the cost of manufacture. Bump stocks were a poor substitute but the only legal way most people could afford to approximate the same experience.
No, I don't have one either. Have shot one, though - burned through 5 boxes of ammo in pretty short order and heated the barrel hot enough to cook on. Fun? Yes. Accurate? Nat as bad as I expected but nothing like I'd shot with an M16, M3, and Thompson.