I suppose just to keep track of all my daydreaming plots and scenario coherence initially. Then, it became an outlet for my pent-up creativity from a boring technical writing day job. Ever sat down a read a 20-page-long academic report? Yea, that. Not very fun to write, and quite frustrating too.
And, I dunno, writing is just fun to me, and I love a hot cup of coffee by my side as I write. Something about it is just relaxing to me.
Inspiring others. When someone lets me know how deeply my work affected them. When I see their strong reactions to provocative moments. The affirmation that what I am making matters. The immensity of the confidence gained knowing I can do it again.
I have an endless amount of ideas in my head and I've always been terrible at putting them on paper. Just trying to fix that step-by-step. Incidentally, I'm much better at being a Game Master for TTRPGs than I am at writing.