Occasionally I’ll see posts deleted right after they were posted or delete my own post right after posting it. The most common reasons for doing so are:
Someone was ninja’d with a much better response and their own response is redundant (even though Discourse pops in new posts real-time, you can’t see the latest posts at the bottom of the screen with the post editor coverting that area of the screen)
Someone had last-second thoughts about their post, decided it was too attacky/disruptive/etc, and decided to delete it
Someone rushes to post only to read the OP afterwards and find their post’s main question had already been answered in the OP / they misunderstood the topic based on the title and their post is actually off-topic
There can be a variety of other reasons, but those have been the biggest three I’ve noticed on our Discourse forum. They’re deleted shortly after so they don’t exactly detract from the discussion, but it’s sort of ugly having “This message has been removed” posts sit around in the topic and have people think “Oooh that person deleted their post they must have posted something bad! Naughty person!” I don’t see any reason why posts within the grace period need to stick around on the thread since they haven’t been up long enough for many people to see them. They’re just taking up space as it is, so it’d be nice if posts deleted during the grace period were instantly removed from the topic.