why is the clutch used in the cars  

Well let me see if I can explain this to where you can understand it! The clutch is the connecting device between the motor and the transmission! you see the motor is constantly turning, so you connect it with the transmission, the transmission turns the drive shaft, and it turns the wheels of the car! the clutch makes the connection between the motor and transmission, by pressing the clutch plate, and the pressure plate together,and the friction makes them kind of bond together with friction,until they are both turning together, thus making the wheels turn! Of course, in this discussion we are talking about a manual transmission, not an automatic!