Writers will give you a lot of reasons why they "do it"; and, no, I'm not talking about "it" but "it", OK?
Speaking from experience, and as somebody who is considerably more aware of his own motivations, including the 'real' ones, than most people (no false modesty here!), I can assure you that the vast majority of those reasons qualify as 'reationalizations' and quite a lot of them are simply bogus. Almost all the reasons supplied though suffer from incompleteness, if only because people just don't know all the reasons that drive them to do whatever shit they're doing. Period.
In the spirit of disclosure then, I'm going to add another reason to the stack I've already unearthed for my addicion to story-telling: because it's saving my sanity.
Seriously! I have a terminal dependency on making up stories. Apart from allowing me to interact (sort of) with a whole bunch of interesting characters, they also let me imagine things that aren't real, but which are like I maybe would like things to be. They also, rather importantly, let me blow off steam about the lunacies of the world and its denizens.
Like, right now, I'm positively itching to rant and rave on about religioids and other idiots who think that they have some natural 'right' not to have their precious beliefs insulted and even ridiculed. I managed to scratch scratch the itch because I happened to have arrived at a conversation where I could let fly through the mouth of one of my characters. Much better putting it in the first draft, rather than wasting time putting it into a blog that'll piss everybody off.
Maybe it'll stay in there, or maybe it won't. That'll be up to my mood at re-write/edit time. But for right now I've written it out of my system, thus contributing to lowering my mental and physical blood-pressure. All good.