When commenting sometimes you want to add to the entire thread instead of replying to a specific post. There is a blue Reply button at the bottom of the thread, but who wants to scroll all the way there?!
Instead, just tap on the reply icon on any post:
Then tap or click on the small arrow in the upper left:
This will bring up a menu where you can choose to reply to the thread instead of the post:
The screenshots are for mobile but desktop should be very similar.