One late return cascades. A car due back at ten and returned at two leaves the next customer at a counter with nothing to give them, so buffer between bookings is a commercial decision rather than an inefficiency.
Condition has to be evidenced at both ends. Dated photographs at collection and return are what settle a dispute over a scratch, and a walk-around remembered by two people who disagree settles nothing.
Maintenance takes vehicles out of service. Servicing, tyres and statutory inspections are predictable, so planning them into quiet weeks keeps cars available when they earn.
The rate is not the number that matters. A vehicle hired eighteen days a month at a modest price beats one hired eight days at a high one, so utilisation per car is what shows which part of the fleet to grow.