I believe the idea is to tag the post being responded to if you use the one under a post. It is supposed to reference the original post in the upper right corner and if you click on that it will open the original post so you can see what is being addressed in your post. UNFORTUNATELY, that is not happening consistently for me. Sometimes it shows the post I’m replying to and other times it does not.
The reply at the bottom is to add a post to the thread without the link back to a specific post.
ETA: I just checked and it is not showing that I replied to you. So it seems like that feature needs some work. I just tagged you at the top of the message to cover my bases. 
This FAQ topic may help.