Differences between a Memo and the Memo Key

Memo Key

The Memo key because it is a bit of an outlier. The only thing the Memo Key can do is encrypt and decrypt private messages that are sent through the blockchain. While this could one day be a powerful feature, today it is not commonly used. If you have received a private message that you would like to decrypt, as always you should use the key with the minimum necessary authorities, which in this case would be the Memo Key. source