If you send a lot of emails on your iPhone or iPad that go to different people, you may want to blind copy some users, or all of them for that matter, so they don't see each other's email addresses. Regardless of the reason, if you forget there isn't a need to delete the email and re-enter it in the appropriate field. iOS actually gives you the ability to move them pretty seamlessly.
That's all there is to it. This is a great tool for times when you need to blind copy someone and accidentally but an address in the wrong field. Try it out and let us know how it works for you!