Basically, if you didn't do this, then people could snoop on your message traffic and even though the payload was encrypted, they would be able to tell that you

Basically, if you didn't do this, then people could snoop on your message traffic and even though the payload was encrypted, they would be able to tell that you received a warning message about having been in contact with an infected person. By sending decoys, everyone gets the occasional such message as far as any attacker is concerned, and no information is disclosed.