When watching the below clip:

at exactly 11:02, FO says that APU is not required.

What could be the reason for using APU on the ground? What good could this bring? EDIT: Both questions concerning depicted situation only -- i.e. a plane has landed in a quite very hot air.

I am a total noob to flying, but I was always more than sure that APU is used in air only. It is used to produce minimum electrical power to on-board equipment. Which means (as I assume) that there is zero electrical power when starting APU and it must be started when flying airplane generates enough airflow (through APU).

What am I missing? In which scenarios (EDIT: for depicted, landed airplane conditions) on the ground APU "could be required"? And how a ground speed of the after-landing rolling plane (very limited to airborne speed), generating minimum airflow, could start up the APU?