Or maybe it is a feeble attempt to annoy people that sign up with foo+service@somewhere.com and then sort it into different inboxes (of course you can filter on other things but + is built into gmail). You can also use it to see who sold your info when you get spam on that adress.
Yepp, and no one really listens to the others, just trying to remember what you did and make sure no one dumps more work on you.