Students get to share their ideas in a way they can be proud of. Blogging is great for this because the posts are read not only by classmates, but also by anyone else who stumbles upon them online.
Blogging promotes the development of good writing skills, so helping students take ownership of the project can help them write well.
Blogging provides students with an outlet for things they may not get to share in the classroom, so giving looser guidelines on topics can bring about more interesting blog posts.