For what purposes do we need "Sign message"? I got a simple but straight question:

Why do we need "Sign message". Also give explanations and examples.
You can sign a message to prove ownership of a particular private key, without sharing the private key or spending any funds. For example, you could sign a message that says "My name is John Doe". You could give this signed message to anybody, and they can verify it with your public key. This proves that whoever has your private key claimed to be John Doe at some point. 

Somebody might sign a promise to deliver funds in the future, to prove that they have access to the funds (if the address associated with the private key has the amount of money promised), without actually sending the money. Basically proving the ability to send money, without actually sending it.