Dumb question - what is the place marker for?
Not dumb at all.
This is a subject I am very interested in, so I leave a placemarker. Then when I go to my ping list, the placemarker shows up as my most recent ping, and the number of posts made by others are also shown. I can revisit the thread periodically to review those posts.
This is especially handy when there are several interesting threads at the same time and I am trying to get some work done as well.