If you have a site that displays your portfolio, a blog can improve the search engine rankings and increase the amount of traffic, possibly leading to more work. So as a developer I would consider it for sure, if you have time. I think the content would depend on your style and how you are presenting yourself. It should be more personal so people can get to know you better, yet still professional so you don’t scare people away. And should target the same market that your site does.