A lot of people may say that it's important for writers to blog so they can get their message out and start building a platform and an audience. While those are great (and important) reasons to blog, I submit that there is a much more basic and far more important reason: you, as a writer, need to write EVERY DAY!
This is something that's drilled into us early on in our writing education. I've heard different minimum word-counts, but it's always been at least 200 words a day. Some have gone as high as 1000 words per day, but that's not always possible for some people.
As you can see (if you're a regular), I don't blog every day. I just don't have THAT much to say. And, to be honest, I have other writing to work on. But this helps me keep my craft fresh. It improves my vocabulary, sentence structure, and cognitive processing. It may not show up in the posts, but it does show up in my manuscripts.
So if you don't blog regularly, I recommend you do so. Not to build an audience, but to improve your craft!