This was my original comment:
If a person is late to shul and someone took his (paid/kove’a makom) seat, he can’t make that person leave. If davening is underway and a guest needing a seat took an unoccupied seat, he isn’t responsible to know or have to move for a latecomer that normally sits there. Otherwise no guest could ever sit down without having to worry he’ll be kicked out of the seat (at which point there may be no more available seats.)