For a long time blogs were seen as vain, and/or as drivel.
Now blogs are driving and interpreting the news.
They are also a way to get your ideas out there… before you’ve finished the book… while your article is still months down the review queue … without having to get anybody to agree that they’re publishable.
It’s also a way to get ideas out that are too small (or too big) for a journal article, too interdisciplinary, or in whatever other way don’t fit into publishing formulae.
Blog posts are nice because you can change and add to them as you learn and ruminate.
It’s also a way to keep track of stuff, including your own thoughts. I end up checking my own blogs when I’m looking for things.
You don’t have to publish everything right away. You can save some of your writing as a draft, and you’ll be reminded of it next time you log in. Much better than saving to a file on your computer where you might never notice it, or think of completing it. Starting a post is beginning the process of getting the idea ready to share with the world, and noticing that you didn’t finish might motivate you to.
It’s a way of communicating with people who aren’t going to read journal articles, communicating more informally, and participating in a more time-sensitive community of analysts, or what are now called “expert communities”, which are now having impact on policy and public discourse.