Airborne refueler. What you're seeing is what they do. Fly in a circle until someone needs gas. So they're on station waiting until someone needs to top off.
True. Good point. They could be holding for landing, burning off fuel. Or they might be holding outside of the area just to stay clear and then go back in if someone needs fuel. If they're refueling helicopters, they'd have to use the drogue or basket and they'd stay low for them as well. That might also be a consideration. Good catch!