I've seen this on a number of threads lately. Posters want to see what happens on a thread so post 'F' so that it stays on their 'threads I'm on' feed.
I get that some threads are more interesting than others, and sometimes a poster doesn't have anything to contribute to the discussion.
But it comes across that the F poster is nosy and wants to know the juicy gossip when it emerges.
Added to that, it can sometimes fill up a particularly fast-moving thread so that it reaches 999 before the OP has the chance to digest the responses to her/his initial post.
An far easier and less 'curtain twitching' way to keep an eye on a thread is to use the 'save thread' function.