Does this already exist and I can't find it?

In a few forums I check regularly, there is a button at the top of bottom of the page that says "Mark Forums as read". This clears out all of the posts that you have not read, but really don't have any interest in reading anyway.

If you don't have this feature.. I'll try to sell it's value to you
  1. Less data pulled for each user. Since you pay based on bandwidth used, this would help the costs to opporate this site stay low
  2. Improves readabilty of forums
  3. Makes happy schlappys