I have had a 'Eureka!' moment and feel quite a fool for not having had it much, much sooner. In all the time I have been using email clients with the functionality to save a draft message, lo perhaps even since the time of UNIX and PINE (I misremember if it had such a facility), I have been an inconsistent correspondent. I sometimes responded promptly to messages; but if I did not, if I procrastinated, then it's highly likely that over time I would forget that there were messages (and their composers) awaiting an answer.
Okay, but so if I start a draft, even leaving it entirely blank, Gmail will save it for me, and I can see at a glance if there is a backlog and how deep it is. You're probably saying, "Well, obviously," but sometimes it takes me an awfully long time to realize these things.