Make a draft email on a phone or tablet (maybe some quick notes) to later revise and send from Outlook for Windows/Mac. Once that was a problem but not any more, as long as you’re using Outlook mobile apps.
Syncronizing the Drafts folder is really useful. You can write a partial message or reply on one device then resume later on another one. We often use it to jot down a response in short form on a smartphone, later on Outlook for Windows/Mac the draft is tidied up and sent.
Outlook mobile apps
Originally Outlook mobile apps for iPhone, iPad and Android left any draft messages on that device only.
But that’s changed for Exchange Server / Outlook.com hosted mailboxes. Now the drafts folder is syncronized with the main mailbox store and, from there, to other connected devices.
There’s no setting to change, it happens automatically.
The Apple and Android email apps can’t syncronize the Drafts folder but you can fake it for non-Microsft hosted mailboxes.
Write you draft message in a shared service (OneNote, Evernote, Word document saved to cloud storage) to pickup later from another device. Or write the draft message and send it to your own address, the draft will appear in your Inbox (and Sent Items folder) to copy later.