Simple fact is that in theatres, even more so than clubs & pubs, getting the cleanest, clearest controllable volume is a must. It makes everyone's job so much easier when using IEMs and clean preamp feeds to the desk. All settings can be saved and duplicated each night with just slight adjustments out front on the PA for the room ambience.
I speak from experience having done an extensive theatre rock show tour myself with amps and then other theatre shows with silent stages/preamps.