When sending Sales documents (Quotes, Orders, posted Invoices and posted Cr. Memos) by mail, calling the actions opens the Email Editor (page 13). Here the user can enter a sophisticated mail body, use attachments etc. as in a "normal" mail editor.

Sending a Purchase Order to your supplier/vendor is a similar common use case as sending an Order Confirmation to your customer. But when you try to send a Purchase Order by mail, just a simple request page is opened without the possibility to enter text or use attachments.

I'm suggesting to make the Email Editor known from sending Sales documents also available for sending Purchase Orders since this is the most common Purchase document to be sent by mail.