As you know
An interface contains only the signatures of methods, delegates or events. The implementation of the methods is done in the class that implements the interface.
when you implemnt interface You are very much sure ,that the method in the interface is implemented in the class.
You can say Interface is a class which contain all unimplemented methods taht are implemented in Class that implement it.
Suppose you have a project with lot of classes and each class have no. of methods.
So , How you know which class implement perticuler methods.We make interface for each/some classes and implement these interfaces in that class.