Message integrity means the data should appear at the receiver exactly as it was transmitted — with no changes during transmission.

Message authentication lets the receiver be sure of the sender's authenticity and that an attacker has not transmitted or modified the message.

Why we need it: encryption protects against passive attacks (eavesdropping), but a different need is protection against active attacks (falsification of data and transactions). Message authentication verifies two things: that the contents have not been altered, and that the source is authentic.