They're not. Lets say your name is Dave Smith and you join mailbox with the username/address davesmith@mailbox.org
If you wanted to join (I dunno) a mastodon instance but wanted to easily identify it and were also happy for the Admin to know your actual email you could use davesmith+mastodon@mailbox.org
- that's an extension.
But if you didn't want the Admin to know your email you could create an Alias in mailbox so for example mymastoacct@mailbox.org
- as long as no one else was already using it of course.
So if your actual primary address includes your name, an extension will always reveal it, whereas an alias can hide it.