Signing messages with cryptographic keys has been a thing before cryptocurrency even existed.
It’s widely used as a way to verify that you own an address (to know you have the private key without having to tell the key to anyone)
Metamask uses this to connect with web3 sites and to have sites verify you own your address.
The only thing is, you cannot do this manually yourself. (On other wallets, you can)
There should be a feature where you can sign and verify your own and other people’s messages manually.
This is useful for proving that you are you, like if you linked your wallet address with a social account, and you need a way to prove it’s you when you don’t have access to your account.
It would be nice to be able to validate and sign with metamask on your phone and computer