Any aircraft engine supplies a propulsive force by capturing a mass of air and then accelerating that air. It may be a propeller doing the job, or it may be a gas turbine (we call it a jet engine). In any case, a pressure increase is needed to force (accelerate) the air mass aft. Thus, the need for a compressor. The prop is a compressor; the fan in a turbofan is a compressor; and so is the compressor (doh!) in a straight jet or a turboprop.
