I've lost count of the times SET aircraft fail to be ready at a time I could depart them. I normally then log the difference between actual airborne time and when they should have been airbrone and it is usually 5-10 minutes. Additionally the opportunity to use the runway can be lost, delaying every subsequent departure for for the rest of the day until runway utilisation is not limiting movements. One aircraft not ready at their base could cumulatively cost their company (aircraft that follow) an hour of extra delays at a busy airfield.