The RWR is a subsystem as well
As are the Dispenser, Jammer and MWS. The EW system is the one you can have in OFF, STDBY, MAN, SEMI and AUTO, right next to that turning knob you can turn on and off the subsystems.
I guess you could say that the RWR display is used for MWS indications, but it's not really 100% correct. Guess it's also because it's a little outdated system that doesn't support full integration.
IRL I work with an EW suite that has RWR and MWS actually integrated and displaying it's info on a Tactical Threat Display. It shows more information compared to the A10's EW system, and it also provides voice cues.