RB does everything PT does with the added functionality of BIAB realtracks and multiriffs etc. or if you prefer, PT is a 'lite' version of RB in its functions and GUI.

PT is the ancestor of RB and it is a little slimmer and lighter than RB so some of us - like me - have used it in the past as a lightweight alternative to RB, but gradually i've stopped using it. If you have RB you don't need PT.