Why are Pre-Molar Extractions necessary for Jaw Surgery dealing w/ Bimax-Protrusion?

Hey,

I have Bi-Max Protrusion with a recessed chin and my surgeon and ortho both said that I need pre-molar extractions (which I got), of the first four premolars. Why was that necessary to do in layman’s terms and why couldn’t the surgeon just do that during the surgery? I’ve asked my surgeon and ortho but I’m still having trouble understanding it. I’m getting surgery to fix an open bite, mouth breathing, & gummy smile. Won’t the premolar extractions fix those issues?