Real Tracks will enhance your song(s) because they are actual recordings of the instruments rather than computer generated. Granted, the computer will be a large part of the final product, but I have not heard midi instruments that sound as good as the real thing (I suppose there are some, I just haven't heard them). For practice purposes I don't think it will change anything about your fiddle playing - that is all you. But for me, when playing the exact same thing along with BIAB RealTracks versus midi, it is just much more fun and better sounding overall with Real Tracks. It is like having some friends over and jamming. The only things missing are beer and laughter (and an occasional mistake).
There will be a slight learning curve to get BIAB generated songs to sound right, but basic stuff is fairly easy. My limited experience with Bluegrass is that it is pretty simple most of the time in timing and chords, and that makes it easier to be successful with BIAB for a new user. Jazz guys are at the top of the list from my standpoint on complexity of the music, but BIAB is rooted in Jazz and handles it very well.

This is a long answer to what should be a short one - get BIAB (with as many real tracks and styles as you can afford - the UltraPak is a good one). It is an addictive program and will improve your enjoyment of playing music greatly.

Good luck and welcome to the fold.