Another good use of draft is to be able to create a PR that has conflicts so that other can see them and ponder them yet have no desire to ever merge the PR just yet even if conflicts are fixed.
My enterprise instance just got this feature and it does't behave that way. I had a merge conflict on a draft that wasn't reported. The block that normally shows the merge conflicts was replaced with a ready for review button.
+1 : I often forget to mark a pull request as a draft while creating it, and it would super convenient to have a possibility to change the status back to "draft".
+1 I just stumbled upon this thread while literally googling "How to revert PR back to draft". This would be so useful.