You start a blog to attract people to your project to help out. Having a blog is free marketing for your project. You can post updates, things you learned, and

You start a blog to attract people to your project to help out. Having a blog is free marketing for your project. You can post updates, things you learned, and advice for others. Your blog can link to the finished project, or if not finished can ask for beta testers to test it out so it can be finished.