I came up with the idea when I was thinking... "I need to do some more Metaltree 2nd edition, I wonder where I left off... oh shit it is not easy to find out where those posts are." or "I planned to do each Ayreon album and I've only done one... I wonder where that post is?"

Then I thought how much easier it'd be if all of the metal posts are in one place... then realized I could say the same thing about many other types of posts.