They're not about rationality, they're more organized this way, I have a smaller and more selected audience, and don't have to worry as much about wasting peoples' time or sanity, thus making it easier for me to braindump. A big part of why I'm writing is so that people I work with at SIAI can examine some of my intuitions, since I'm not good at explaining them on the fly. It also makes me appear more productive than just reading a bunch of books and storing them in my brain, waiting for big insights.

Spewing out lots of content also seems personally useful since I'm not that good a writer, and I strongly dislike being not-great at things.