The RON instruction does two things in my view. First it is a clear indication that the controller is no longer taking direct responsibility for the route that the aircraft is flying (and, consequently, for instructions which provide the requisite obstacle clearance). Second, it can be supplemented by instructions about the point that the FPL route is regained. Without this supplement the controller doesn't know what what the pilot will choose to do because various interpretations exist - do a search, the topic has come up before..