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.
So an automatic "I am a lazy piece of shit and think my time and convenience are worth more than yours" warning? I guess that's useful.
That just means the person sending the message didn’t bother to proof read their message before sending. And you don’t need to be on an iPhone to mistype a message.
A simpler explanation was that it was a shameful advert injected into the end of people’s emails.