Interfaces are used to define a contract for the classes. It has its own importance when it comes to defining standards.
Consider the following example:
Cars have some standard and their basic operations are the same. Each car should have operations like startCar, moveCar, stopCar etc. Toyota, Mazda and Honda are cars and each has its own way of starting, moving and stoping. So each has to have its own startCar, moveCar and stopCar method.
How to enforce