The tug could be more like a powerbank. It would not physically pull the aircraft up but assist by providing power. The aircraft could launch on its own but it is advantageous to use a tug because this increases the range or energy reserves that the aircraft itself has.

(outline of a launch: aircraft and tug are connected by cable before launch, both lift-off, gain altitude, cable is detached, aircraft moves on and tug returns to launch site)