What is a function of the clutch in the vehicle?

The purpose of a clutch on a vehicle is to disconnect the drive of the engine from the gearbox (this would be required when the vehicle is stationary or to change gear).

While the engine is running the crankshaft is continually turning, It is the crankshaft that turns the gearbox and via the gearbox the road wheels. Obviously there are times when the engine is running but drive is not required,

The clutch is the mechanism by which this is achieved.