You don't have to ever use it if you don't want to. It is an optional feature. For example, if you click "Mark forums read", it would tell the system to mark every item read.
Then if you click "New Post" real quick, it will probably show 0 new post.
Lets say you waited an hour, then came back and clicked new post. There might be 50 new post since you last checked. You might skim all of the new post to see if anything is of intrest, when done, you could click on "Mark Forums Read" and clear all the post again.
The forums are so active (see attachments) and that is sometimes is tough to keep up with the threads and post, this feature could help you keep up on what's happened since you last Marked your post read. Hope this helps!
see attachments for more detail.