Fantastic Four v3 67-70 Tabs

[MushroomMan]

I was wondering why ff v3 67-70 has tabs? Here is a picture of what I'm talking about.

http://img23.imageshack.us/img23/813/fantasticfour496.jpg

[Iron Maiden]

I do remember other books having those weird squares but never knew why. The trade of "Unthinkable" doesn't have them though.