When you add an ID document that contains an address, we send that address along with other details from your document to an identity verification provider, asking them to confirm whether your address matches their records. If it does not, the check will fail. We do this to verify that you really do live there. Note that the identity verification provider does not retain the details we send them: they simply check for a match, and return a yes or no answer.
If you add your address manually you can choose not to have your address verified. You can still add your address if you choose not to verify it, but your address will be marked as ‘unverified’. Unverified addresses can still be shared, but you may find that some organisations only accept addresses that are verified.