A 'walkthrough' is document that helps you get through a game. Zelda, Half-life, MYST, all can be confusing games. But with a bit of help, they become more managable for those that don't want to spend a lot of time on the game, but still want to finish it.
I've always been impressed by the voluminous walkthoughs out there. Sometimes there are documents that are well over 200 pages long. All written by fans, and made available for free.
So, it is no wonder that Wikibooks is a good place to put these documents. Why not let others share in the work? Or if you know something that is not in a particular walkthrough, it's easier to edit one already written, than write your own.
But, while I was searching through, I found a 'walkthrough' for Space Invaders. Really. Space Invaders.
I'd like to meet the person who asks for a walkthrough of a game where you have a joystick that only moves left and right, and a single button that fires your cannon. :)