Why write? I suppose there are many reasons: sharing your opinions, persuading others of your positions on issues, journaling through personal challenges, getting things off your chest, creating something beautiful, making a name for yourself, getting published, perhaps even generating income (good luck with that!).
For myself, I find writing to be my best way of thinking. Perhaps it's just me, but I have the kind of butterfly mind that flits from thing to thing and doesn't stay focused on one task or idea for long. If I merely muse about something, my thoughts tend to remain vague and unformed. Only by forcing myself to create a concrete formulation of what's going through my head do I hone the idea sharply enough for it to be useful.
As an example, I always have a backlog of a few dozen topics I'd like to explore in this weblog, but they remain hazy notions until I write them out. It's only then that I clarify my thinking enough to make what I write part of my mental furniture, as it were. Naturally, the furniture might get moved around further as I have new experiences or if I wish to formalize the relevant thoughts, as I might if they will become part of one of the books I plan to write. But step one is always to put things down on paper.
(Cross-posted at philosopher.coach.)