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.
If you've got multiple email accounts set up on your iPhone or iPad, the Mail app will automatically reply from whatever account an email was sent to. But what about when you're composing a new email? If you've got one account you use more than others, you can set that as your default account for composing emails. If you ever want to send from another account, you can still do that too.