Why do this?

1. People reading the blog have similar interests
2. Links to your site are considered “authority” links and give you credibility
3. If you do it in more than one place, you are considered an authority

There no cost to post a comment. But plenty of opportunity.

One caveat.. don’t look like a spammer. Make sure you have read the post and comment about it. Never say “great post” rather “you said X and I’d like to add Y” and then put a link to your site.. but not a sales pitch.