Quora: Why don’t aircraft turn off their engine when waiting near the runway before take-off? I have observed they wait more than 15 mins near the runway when there are more aircraft landing.

An aircraft engine has multiple functions. It is used for generating electricity to power the various systems in an aircraft. It also draws in air that’s needed inside the cabin. If the engines are switched off then the pilot has to turn on the APU or Auxiliary Power Unit — a generator under the tail of the plane. When an aircraft is parked at the gate, it draws its power from the ground power units. It also takes at least 3 – 5 minutes for the APU to power up the engines if these are switched off. That is a lot of time in an over crowded airport like Mumbai — which has only one functional runway.

A more detailed answer here on Quora.