For me
BiaB - very quick to try new ideas and different chord patterns or different styles etc (it regenerates on the fly, which is faster and the default is to regenerate all tracks every time you hit play).

RB - used for when you want to start drilling down and adding/editing things that either BiaB can't do, or you simply want more tracks to experiment with, or more synths, or more FX, or routing options, or only change this track from this point to this point, etc
Also its default behavior is it only regenerates the sections you tell it to (and also writes a backup of previous version so it can Undo).

BiaB is very different than RB.
I'm glad both exist.

If you want to jam,experiment, or write tunes, BiaB is faster/better.
If you want to start getting surgical and control the result more, RB is the next step.

I do not work here, but the benefits are still awesome
Make your sound your own!