Blogging takes time and only in rare occasions the bloggers make any money, so why do we do it? Jeff Atwood has a good theory. But I like to think that it is more than an "egomaniac action".
Blogging is, for sure, a good way of promoting yourself. In our blogs we put our opinions and also part of our work. A blog is a good way of showing future employers which kind of person you are.
A blog is also a way of networking, a way of sharing your knowledge, but also a way of learning from other people comments.
And the same can be said for the blog readers. I invest daily near 1 hour reading blogs and news. It is a great way of learning from other developers and stay updated.
Our profession/hobby is always evolving really fast, and blogging is, so far, the best way I have found to be in the loop.
See you soon.