The idea behind both of them is to do automation for you, so bass/vocals' perceived volume remains the same throughout the whole song.
Now, I wouldn't find the Vocal rider usable too much for me, because usually I manually ride the vocals for better impact, but the Bass rider is apparently so much useful (judging by the clips), because you don't have to manually ride the bass to stay audible (or, so it doesn't drop out on some notes, which is normal, even if you comp+limit it to hell and back). And as we know, a consistent bass, which doesn't jump all over the spectrum is the key for tight and solid low end in the mix.
This is the only alternative to manual, boring bass riding via automation in your DAW.