When you are playing a game with no Game Master/
DM/judge ensuring fairness, you need some kind of standard of play to make sure both sides understand how the game will work. That standard is the rules. Without rules you aren't playing a game. You're just playing, in the manner of children in the sandbox.
If you let the background decide the rules, you will never have consistent rules because the background is written from each army's perspective. The background in your Codex Space Marines is not going to tell you a bunch of stories about your space marines getting their tin-plated asses handed to them. The same thing with my Codex Eldar. So when we meet for a game, without a common and objective ruleset, you think your space marines need to always win because your background says they do and I think my Eldar need to always win because my background says so. And then we waste hours arguing about background rather than actually playing the game.
Now, that's not to say the rules as written in the rulebook are the only rules you can use. There is nothing wrong with making house rules, home-made special characters, vehicles or units, scenarios, or even re-writing entire codicies if you want. And these things don't have to be fair in a strictly play-tested and perfectly costed sense (we don't even get that with the official product!

) But they have to be fair, meaning the opponent needs to understand how they work beforehand. If you've come to an implicit understanding with your friends about how the rules work and that understanding is different from the rulebook, that's fine.
RAW is not the ideal alternative to just making things up as you go along a la the sandbox. But it is a very efficient way of making the rules understood to both players without hours of discussion beforehand or playing several prior games together to build an understanding. When playing with strangers, like at a tournament,
RAW is usually the most practical way forward.