@lilaL public unecrypted messages are ... public, so you can see all mentions, when they happened etc. It's secure in that you can be sure a signed messaged came from a particular person (or rather their computer), and you'll know if someone steals their key and posts things because the system doesn't support forking an identity.

with encrytped messages, you can't see who they're addressed to, you can only try and decrypt it with your key, and if that works, it was for you!
This is an advantage over e.g. encrytped email.
You can still see when people message relatively, but with enough private messages going on things get harder