Back in the day, it was useful, as in, "Expect awkward phrasing and unintended effects of autocorrection, because mobile device. This message doesn't necessarily reflect the intent of the sender." (Considerate users would/could edit the signature to something w/o a product name in it.) Nowadays, this is pretty much the norm and no explicit warning ist required anymore.

When they added this it was extremely useful - it signaled that you could afford an iPhone. It was really easy to delete, yet people not only didn't, but they would go out of their way to respond from the iPhone just so that they could plausibly have this status symbol on their email.