To me that's only part of the truth. I write because I like it sure, but it's also very unmotivating to just "scream in the void". I want to share idea because I want to hear other opinions on my ideas. I want conversations, not monologues.
I have literally millions of words of writing that no one else has seen. Some of it is a rambling mess, some of it is fairly polished. But having done this has served me extremely well in many areas of life - I am more self-aware, articulate, etc... THAT is the motivation, not whether people have seen the ideas. Perhaps someday I'll refine it further and share it publicly.
Though, I'm regularly drawing upon it all when I have conversations - be it in real life, or in places like this. Why does the "conversation" have to be in the comments section of your own site?