Why handles of motorcycle are covered with a rubber sheet with spikes?

This is called handlebar grips and the function is to provide material that is easy to hold in your hand and that partly absorbs vibration. It would not be comfortable to hold the bare metal of a handlebar in one's hand. A motorcycle driver needs a grip made of material that increases driving safety, improves the handling of the bike and prevents his hands from gliding too easily off the handlebar as he steers the bike. So the function of the grips is borh to increase safety and the comfort of the driver.