I don't think it is anyone in particular, it is hard for everyone to follow threads where there is an extended subthread. This makes it easy to follow the discussion you care about when a subthread goes on a tangent you don't care about (or flamewar, etc.). I am super excited to see it and would have missed it for a while if not for this discussion.

Edit: Oh yeah, there has been that tiny collapse button for a while, I somehow never got into the habit of using that one and forget it is there. I wish there was a setting to use the words "up vote" "down vote" and "collapse" rather than needing to pixel hunt :/.