What is the major importance of a flywheel in the engine?

It Provide constant power output to the drive shaft by the torque of the flywheel.

Providing continuous energy when the energy source is discontinuous

Being very simple in answering,FLYWHEEL is a device that stores energy when in excess , and releases it when it is deficient .

Hopefully the link below will define your question's answer. Thanks

https://www.quora.com/What-is-the-purpose-of-a-flywheel-in-an-automobile-engine