I assume MN do that when an OP's update with pertinent information is posted further down the thread, but everyone is replying to the original OP - it helps people read important information. (You can see that pinned post was actually posted later on).
MN have occasionally put a note saying 'please read all of OP's posts' in the title or OP because people pile on to the first post without reading the rest. In one case everyone was having a go at OP presuming her pregnancy had resulted in a live baby, when it hadn't. So clearly that was worth clearing up.
I'm just speculating though - I haven't tried posting my own thread to try and do it!