The restriction from entering a waypoint after the FAF has more to do with required navigation performance (RNP), and how an RNAV/GPS approach is designed. All aircraft certified for RNAV/GPS approaches are designed to maintain an RNP of 1.0 NM prior to the FAF. RNP after the FAF is reduced to 0.3 NM. Inputting a waypoint after the FAF "corrupts" the approach stored in the Nav database and may keep it from changing to RNP 0.3 after the FAF. Not a big deal if you are on a visual approach, but you would be illegal to fly an RNAV/GPS approach in the weather. Deleting a waypoint after the FAF may not necessarily change the RNP but depending on what is deleted, it could affect how the both the lateral and vertical/glide paths are calculated and potentially compromise obstacle clearance during final approach. All this is dependent on the type of aircraft/avionics package and the the specific approach flown. Remember, not all RNAV/GPS approaches are straight-line. from the FAF.