a full bypass guides air around the engine, as well as providing a seperation between the engine and all of the bits in the fuselage, creating a clean install. i have also seen bypass's protect engines in the event of crashes. one that comes to mind was a planes plus F-86 that went in. in the crash this black "football" looking object squirted out of the wreckage and bounced about 30 feet down range. it was the turbine in the bypass and outside of a bit of grass, was fine.