Support CC, BCC or multiple TO
I think this is pretty self-explanatory.
Especially since MailPoet can be used to send transactional emails on behalf of the entire website and other plugins.
Many times multiple employees need to be notified when a form is completed - This is usually done by sending the email using CC or BCC or multiple TO email addresses.
This is a very common use-case (used on nearly every single client website)