That’s a feature for power users, I suppose. It allows to resend an existing email message untouched. So, it’s not the same as mail forwarding, but similar.
Because it’s similar to forwarding the Resend option is placed in the Forward menu.
The bounce does not require a complete mail composing page, so we use a simple in-place dialog with a few form inputs. From, To, Cc, Bcc information will be appended to the original message as headers with ‘Resent-‘ prefix according to mail standards.