Oh brother. This is one of those rare threads that to respond at all would require a good sized thesis. Not today, thanks.
LOL! Right you are, and inasmuch as I have just popped another cap on a particularly good IPA I'll make a shtab at it...
I think I get it, I really do, although I'm not a writer on the order of a Nabokov or a drinker on the order of Amis. But what they're looking for is The Zone, and alcohol helps them get there and stay there. Other stuff does too, but tends to be too severe for longevity (i.e. King's cocaine, and, well, we'll get there).
It isn't just writing - musicians, especially improvisational musicians such as jazz or rock musicians, know all about The Zone. It's where the self and the mechanics sort of fade and what comes out seems only to have muscle memory between whatever dark recesses of the brain from which it bubbled up and the chosen instrument. When you're in it stuff just comes out, and later if you're reading it or listening to it you tend to say "Wow, that's good, who was that?"
With musicians it's alcohol and especially heroin. I don't know why but I can make a case from the artists who have died from those temptations, (and the not inconsiderable number who conquered them and ended up dying in some sort of aircraft crash afterward, but that's a topic for another day). If you're a real ego-head you assume The Zone is you and your talent, but it isn't, it's only a place you visit if you're really on. And it can be hard to find, as a lot of superb writers find out to their utter devastation - Hemingway, for one, who blew his brains out over it. You drink, or you shoot up, because it helps you find The Zone, or later, tragically, because it makes you think that you have when you haven't.
That isn't to say that anything like alcohol is required for brilliant writing, or music, or whatever. But if you start to think that it is, it is. And coming out of it means that the individual involved feels that he or she must choose between a life of mediocrity or a another taste of artistic achievement that turns out to cost the artist that life, even if by then the achievement is only illusory. It's a double addiction, and a profound misplacement of self, and it's a little frightening to contemplate.
Fortunately for those of us not cursed with that sort of evanescent talent, there's still beer. (Sound of BtD falling off his chair and snoring on the carpet).