In addition to those categories, speedrunning glitch hunters tend to gravitate to participating in these projects as well. E.g. the Twilight Princess decomp was started primarily by and for the speedrunning community.
It's also the endgame for romhacking, once a game is fully decompiled modders can go far beyond what was feasible through prodding the original binary. That can mean much more complicated gameplay mods, but also porting the engine to run natively on modern platforms, removing framerate limits, and so on.