It has been suggested that when sending a Quote to someone, you may have the scenario where someone else has asked for a copy of that Quote (either from a different Division or another Contact at the same Division). This is common in scenarios where you may have multiple people competing for one Quote/Project.
In this scenario you could do as I've done in this loom https://www.loom.com/share/3a049bfb200246698de7fe00f9cfc94d and change the To email address (or CC/BCC) however doing this still stores that document record that is created under the Quote that you are creating it from. It would be useful, if sending emails from the CRM to other email addresses than the Contact that the Quote is under, for the document to be created both under that Quote record to be able to see the history of emails sent related to that Quote, but also created under the Contact in the CRM which has that email address set as the primary email address.
I realise that if you were to email an address that wasn't in the CRM as a Contact record, this wouldn't have a Contact to save it against so could just leave it saved against the Quote.