RB allows more than one additional audio track to project.
RB makes it easier to edit, and regenerate sections of a track.
RB allows multiple routing options for FX and grouping of tracks when mixing.
RB makes many of the jobs easier when trying to get the song jut how you want it.
RB allows real-time FX use while recording.
RB allows 'multi-riffs' which let you pick through numerous generations of a track and assemble the parts you want into a finished track.
RB lays things out linearly, start to finish whereas BiaB uses verses/choruses that repeat.

There are many things RB does that BiaB doesn't. There are things that BiaB makes quicker and easier. Many users start out in BiaB to get the main idea worked out, then move to RB to make the additional changes, edits and record additional tracks.

They give you both of these tools, so it is worth learning them.

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