As a developer you will be doing a lot of different things. Sometimes you will run across a problem and it may take you some time to work out the solution.
By blogging your experience and solution you may help someone else in the future. I have actually found that my own blog posts have helped me in the future. I tend to forget things.
Yes I think having a blog is a very good idea for a developer. I am speaking from experience I have had one for five years now.