Maintaining a consistent fictional world to create a shared sense of community between authors and fans. What else would it be for?
I maintain that this is not at all important for playing D&D, as evidenced by the fact most people play homebrew anyway. It is important for selling products, certainly. Some people seem to be arguing that anything that deviates from canon is somehow 'wrong'. That is just plain stupid.
Forgive me for quoting (well, paraphrasing) Princess Bride, but, "Canon. You keep using that word. I don't think it means what you think it means."
Oh, I know what it means alright. I'm arguing its importance (or lack thereof) and role in D&D.
While fans can argue about what is canon and what is not, due to shared worlds like the Forgotten Realms often being riddled with inconsistencies, reboots, and such, you don't choose what is canon. It's not what's useful for your game, or what you like versus what you don't. It's the official "truth" or "history" of a fictional world, as delineated by its creators, in this case, WotC.
Which, as I said before, should have absolutely no bearing on anyone's D&D game. Canon is one possible (murky) truth out of an infinitude of possible truths. You can call it "official" if you like despite the inconsistencies, reboots, etc. but again, if it can't be modified for your own use, then it serves no purpose, and if it can be modified, then it isn't really canon anymore, is it.
When running your home D&D game, you should absolutely keep what you like and trash what you don't. But that's not you choosing what is canon and what isn't, that's you choosing to ignore canon and go your own way.
Certainly.
Well, okay, that's fine. Than why are you arguing about canon if you don't care to follow it?
If my statements above don't make my point clear, then nothing will.