Why do we need to use interface?

Your question doesn't provide much detail so I'm going to go by what you meant as best as I could do apologies if I get it wrong.

If you meant "Why do we need to use a user interface"

Short Answer: A User Interface or known as a UI is needed for the operator of the device to interact and communicate with the device they are using.

Or if you literally meant interface,

An interface is two or more components that can exchange information that can work by software, hardware, and or humans.

You need to use interface because without an interface u cant interact with your electronic device