It was a little easier this time, because it seems quite similar to the reason why I lost drafts before: Draft is no longer automatically saved after you cancel replying
I’m not sure the save button would have helped in this particular case.
But losing drafts for whatever reason is never fun. And the fact that Discourse doesn’t save drafts until you’ve entered 20 characters means that when I start quoting other posts, I can easily lose everything - because quotes don’t count toward that limit. So I can add one or even several quotes I want to refer to, and as soon as I close the composer, everything is gone. There’s no warning that, even though there may be hundreds of characters visible in my composer, nothing will actually be saved. That’s where this button was helpful: it indicated that Discourse considered my post long enough to start saving it.
I often close the composer for various reasons - for example, when someone replies to me in another topic and I want to answer there first, or when I want to test something before continuing, or simply because I prefer to continue on my laptop instead of the tablet. In all these cases, the quotes I’ve already added are lost when I come back, and I have to search for those posts again. That’s exactly what I’d like to avoid. That’s why it has always been important to me to make sure that my draft is being saved.
Of course, that doesn’t protect against bugs, but at least it protects against the normal behavior that has caused me to lose drafts before.
Edit: I tested on a forum which is on a version before the discard button was changed. The missing “saved draft” option would have indicated that something was wrong with the draft. When I started my draft and closed it using the discard button, “saved draft for later” was shown:
The same was shown after I navigated to a different topic.
But after I used the reply and then the cancel button, “save draft for later” disappeared:
So checking that the button was shown would have correctly indicated that my draft wasn’t saved.