For my purposes, RealBand allows me to make song arrangements that are not limited to the 5 main instruments of BIAB. So I can have 3 guitars, pedal steel, piano, strings, drums, bass, fiddle, etc. up to the limit of RB (I think 48 tracks). I can then cut and paste sections, delete sections or move sections of each instrument pretty much at will. Very useful in my opinion for really getting the most out of the many Realtracks and midi sounds that come with it.
I use BIAB for building the basic structure of the song and often BIAB is adequate for a given arrangement so I don't use RB on all songs, but it sure is nice to have once you learn how to use it.
Good luck.