The purpose of a flywheel on an engine is to allow the starter to turn the engine over so that the vehicles engine starts.
Without the flywheel the starter would not be able to turn the engine over and your vehicle would not start.
The flywheel is large in diameter to allow the small starter motor to use less energy to turn over the engine which would require more energy without the flywheel.
Basically the Flywheel is a rotating mechanical device that is used to store rotational energy to allow the engine to be turned over easily.