You may make friends and get more exposure for your own blog, but the best reason to comment on a blog post is to improve the blog.
How do comments improve a blog?
Continues the conversation.
Encourages the writer.
Lets the writer know more about the audience of the blog.
So let me know more about you in the comments. Do you write a blog? Or have a website? What are your favorite blogs? What are your favorite websites? Do you have questions?