Here is why:
- It says that you found the content helpful / interesting / amusing enough to take a minute and leave a comment
- It encourages discussions amongst others who comment and also between the blog writer and the readers
- It shows new visitors to the blog that some people do read the blog and leave a comment, which might encourage them to do the same
- Everyone feels good about being appreciated. In blogging world, comments are an easy way of showing this appreciation of the content someone's providing
- It's a give and take thing. If you leave a comment on someone else's blog, it encourages the other person to leave one on yours.