This might not hold true for your vitriolic rants about your ex-girlfriend and similar, but blogging technical content is great. It’s good for the community in general when there are good howtos, explanations etc… and I’m sure a lot of people like the feel of being read by many people, but it’s not just that.
It’s often said that a true test of whether you understand something is whether you can explain it to someone else. Even if you were to erase a blog post the second you finished it, chances are you will have learnt something. I always do, even if I’m writing about something I already know well. Putting something in the form of a coherent piece of writing pushes me to explore all the edge cases and ramifications which, even I was aware of, hadn’t fully thought through before.
So go and write something interesting and do yourself and the community some good. Pick something interesting you worked on recently, a common question or misunderstanding, anything will do!