|Why'd they even bother writing about this guy?|
First I'd just like to say that I'm not completely against WAAC gamers nor do I think that everyone should make their lists fluffy and whatnot, just that anyone that completely disregards the lore is a complete and utter moron.
So why is it so important?
Well, it's pretty fucking interesting for starters. I will admit that there's some pretty boring lore here and there, but for the most part it's fantastic.
But the major point I think is that there would be no 40k without it and there wouldn't be a game for wankers like all the YTTH fanbois to play in the first place. If there was no fluff/lore, there would be no Space Marines or Tau, no Orks or Tyranids. Instead you would have Army 1, Army 2, Army 3 and so on, each with different point costs and stats. And while we're at it, why bother with different models or terrain? Instead the armies all have the same models, but Army 1 is blue and Army 2 is red! And instead of rules for buildings, it's rules for cardboard boxes and books. The game can be called "Sci-Fi Miniature Tabletop Game". Sounds fucking great!
Seriously though, who would buy something like that? No one, that's who. And that means that Games Workshop goes out of business and there's no 40k for YTTHFB to bitch about.
Aside from the fact that the lore is an integral part of the hobby, there are a few other benefits it has. First, it can help inspire one to make their army look cool based on stories, it gets people to buy other armies because they like their lore (thus making GW more money) and it makes your army far more interesting and adds a lot of character to it on the tabletop, which ultimately enhances the gaming experience.
But why should the fluff play a part in how the rules are written (e.g. limiting the armies an army can ally with)?
Because fuck you, that's why.
On a slightly more serious note, it's because it breaks the game. Every tournament would consist of the same two armies allied to each other because it's the most broken combo available, and anyone not playing these two armies gets destroyed in every game they have, don't enjoy it at all and never go to a tournament again. It takes all the fun out of the game and removes any serious thought that needs to be given to writing a list or who to ally with.
People who legitimately ask this question are the people that ruin this hobby for other people, the kind that take the fun out of everything and paint their armies to the standard three colour minimum so they can score some extra points in tournaments. If you're one of these people, go set your three-colour army on fire and choke on the ashes.