Why We Need Interface?

Question

I have a doubt on interface. My question is why exactly i need interface? i read few articles in the net but none of them explained me clearly.

1.) Every one just say interface is a contract that can be implemented in a class.

2.) They just give an example of Idisposable, IComparable etc.

3.) My question is when i implement interface called ICompareable i should and must use CompareTo()  method i agree, my question is that in CompareTo() method i will be writing the logic. So wy do i need to implement Icomparable interafce? I can write the method directly in a base class and i can use that method in derived calss?