Why do cars burn petrol?

Answer 1

Petrol goes from your gas tank to the carburetor where it is mixed with air. It is then fed to the cylinder, compressed by the piston, and ignited which pushes the piston down. The piston turns the crankshaft which turns the transmission which turns the driveshaft which turns the axle which turns the wheels.

In short, burning the petrol provides the energy to turn stuff and make your car go.