Primarily, I write:
One of my heroes, Socrates, despised books (find quote).-> the same reason for wanting to produce instead of consume.
There are (at least) two ways in which writing things in black and white improves my thinking.
New topic -> new way of slicing the world (quote from Zen or the art of motorcycle maintenance). My mind gets primed to connect ideas in a different way, and I find this process exhilarating. Also, it attracts other's ideas when they read them.
Writing is one of the few tools that gives me the feeling that the knowledge I am building can compound over time. I forget books.
Moving from being a passive consumer to a (small) producer changed how I consume certain kinds of information. When I am receiving some interesting bit of information (say, reading a book, or listening to something), I now have something to listen for, because I know that someday that insight might percolate to some email, some blogpost, some note, some Anki. It has made me more attentive to detail, more willing to grasp the core of the arguments, etc. Producing information adds a certain kind of pressure to change the way in which I consume information. This isn't always a pleasant thing, or one that is good. Sometimes I have to remind myself that I am just reading for fun, or in an atellic activity El plato paratélico.
I write because when I do, the topic of my writing becomes HD. If I write about some idea, the on-and-off meandering around the main concepts makes it crystallize. When I write about human life (could be either fiction or my diary), I get emotionally closer to life, I can feel its grain.
I am a slow thinker. Spoken conversations happen too fast. I get my best thoughts afterwards, in the stairs. I typically need time in order to understand what you have told me, especially those bits that are shocking to me, and I need more time in order to come up with a clever response. This is part of the reason my blog is focused on the people I can have conversations with, see Why I blog.