Originally Posted by abhijeet080808

Pressure is a force on a limited area. Air pressure inside the tyre will keep it inflated. Pressure on the piston will force it downwards and rotate the crank. Pressure in the brake lines will stop your bike. So there you have it, the uses of pressure.

In case of an engine, it gives us a measure of the compression ratio of the engine. Also a good engine will show higher cylinder head pressure compared to one with worn out oil rings etc.