My recurring use case is when I'm starting a PM or topic reply while logged in on one machine, but need information to complete the reply from another machine. Saving as draft gives me an easy way to "transfer" the information (i.e. the in-progress reply) between those two machines.
That said, I wish it retained attachments on the drafts, rather than "saved drafts only include the subject and the message, any other element will be removed."
Because that reality usually determines the "direction" in which I use the draft feature: Whichever machine has the data that needs to be an attachment must be "the second machine" I access the draft from; rather than the machine where I originally create the draft.
I take that back... as I look in my drafts folder here on phpbb.com/community
, it appears that I also have an additional use case for aborted or second thoughts before submitting some "The Coronavirus Topic"