Replying to a post is intended for typing up a message that responds specifically to one post. The reply context will be included in your new post and the person you are replying to will get a notification
Versus if you are replying to the topic generally, there is no reply context included
On a side note, the default behaviour on Discourse is that “replying” to a post directly above yours causes the reply context to be removed when the post is published. I think the default behaviour is confusing because of the inconsistency but fortunately it can be changed in the Settings:
