Blog Directories idea actually came out when most website directories don't accept blogs to be submitted in the website directories. And that's why people create blog submission to collect all blogs contents.
More and more blogs are created everyday. To enhance the promotion of blogs, bloggers usually submit their blogs to blog directories. When a blog is submitted to a blog directory with high page rank, the blog will get a backlink from that blog directory. That's why blogger loves blog directory submission and treat this as a must after having a blog. After every post of new article, you need to ping blog directory, so that it will send a crawler to see if you have updated your blog.
There are many free blog directories where you can submit your blogs, such as: technorati.com, bloggapedia.com, digg.com, blogcatalog.com, blogtoplist.com. For more information about how to get free traffic using Blog Directories.