That time is used by the ground crew to prepare the plane for the flight and go over safety systems. Planes can’t just land and takeoff again immediately. Things can go wrong, and you can’t pull over in the air. Sometimes those delays are mechanical, and some times those delays are shift changes, bad crew scheduling/staffing levels. Airports are pretty fucking efficient over all. From the planes flying people around perspective, not the human shuffling element. We are just cattle to them after all.