I like writing because it forces you to turn vague feelings into concrete ideas. You can barf your thoughts onto paper and then read it with a critical eye, allowing you to evaluate your own thoughts. It helps me to find the gaps in my knowledge, notice sloppy arguments, etc. Sometimes that leads to a better understanding of the topic, but most of the time I realize that I have no understanding of the topic, which is…
I don't like it because I feel my vague ideas "disappear" when becoming concrete. It feels like trying to project a multidimensional creature into 2D; you inevitably lose some of it. In contrast, in my brain I can rotate or pinch the idea anyway I like and even turn it into further ideas.
And, much like the scientific method can take something that "everyone knows", and turn it into the basis for a branch of science, the process of getting this down in paper forces you to document all the nuances in your head, making that final idea even richer and fully consistent.