Why Comment on Blogs?

• Branding / Awareness – commenting on blogs will almost certainly get you at least a brief once-over from the writer, and consistent contributions are a proven way to build relationships with bloggers. That participation can also yield awareness and branding to the blog’s audience, at least those who are consistent comment readers and interactors. Depending on the blog/sector and your goals, this can be a very positive marketing move.
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• Direct traffic – comments with links, especially those that are well-written and entice readers to click a link (rather than just being a random/irrelevant/spammy link drop) will send visits. If the post itself continues to earn traffic, this can even be an ongoing source of referrals to your site/page.
   
• SEO / Search Rankings – some blogs use “dofollow” links in the comments that are approved and may send search ranking value. However, it’s generally my opinion that many of these links aren’t treated as… let’s say “impactfully” as normal links in the search engines’ ranking systems. However, leaving a link that is so useful and valuable that the blog author edits his/her post to include it (something I’ve done many times here on SEOmoz and helped to make happen through my own comments) is definitely SEO accretive.
   
• Second-Order Marketing Impact – many of the effects you might feel from commenting aren’t directly impactful, but instead come later on as a result of the post. e.g. the reporter who follows up on a comment for detail to include in a story, the link to your content that comes from another blog discussing your comment, etc.

The important point here is that comments are of limited SEO value. The real value in blog commenting is building a brand within a community.