On the old vBulletin site, it would display a little check mark next to the thread title if you had previously posted in it. (Actually, IIRC, the check mark meant that you were subscribed to a thread, but since you were automatically subscribed to a thread you posted in, it was essentially the same thing.)
For me anyway, it served a couple of useful purposes. If a thread title that I didn’t recognize appeared with a check mark next to it, it was a tip-off that a zombie thread had been resurrected. And knowing that I had posted to a thread, I could go find what I posted to make sure I didn’t wind up posting the exact same comment again.
With Discourse, there doesn’t seem to be an at-a-glance way to tell whether I’ve already posted in a thread or not. Or is there, and I’m just not recognizing it?