If I didn't write, well, the world wouldn't end. (Though since I won't stop writing, you can't be sure even of that.) Probably, too, it's safe to say that no-one would really notice, and that greater disasters befall humanity on a fairly regular basis.
On the other hand, if I didn't write, the world which wouldn't end would make a good deal less sense to me. The people not really noticing would become less interesting to me. The greater disasters befalling humanity on a regular basis would mean less to me. And for me personally, there couldn't be many greater disasters than if I didn't write.
Writing matters to me, a lot. There's a (probably apocryphal) story that when the Russian space program was in full swing they strapped a man to a bed for a few months to see how his body would react. (somehitng to do with zero-gravity, I suppose.) Of course the lack of movement led to his muscles' atrophy and eventually he died. I feel like much the same process would assault me if I renounced the pen and took up sky-diving instead.
I write letters, and emails, and text messages, and journal entries, and articles, and essays, and short stories, and everything writeups, and soon-to-be-abandoned novels, and unfinished plays, and the occasional god-awful poem. I don't know if I'm any good at it or not (not good in the sense of being articulate - I'm aware that I am capable of structuring a logical sentence, and don't consider it a particularly important gift - but good in the sense of being able to reach people in some fairly fundamental wordless way) but it doesn't make any difference: it's so much a necessity that even if I knew beyond doubt that my best efforts were sub-McGonagall I'd still probably carry on.
This isn't intended to make the whole thing sound more mystical or noble than it really is. It ain't all poetic and deep and difficult. Most of the time it's just - writing things down. But somehow, perhaps because of the imposition of order, the implicit connection between the neatly patterned worlds of fiction and messy reality - I find it an exultation in the good times, a solace in the bad. It's a prayer to the god I don't believe in; it's a letter to the person I hope exists. It's as fundamental to my existence as wearing clothes or talking to my friends or sleeping in a bed. If I didn't write is ultimately too terrifying a sentence to leave unfinished. If I didn't write, I'm not sure I'd be capable of doing anything else.