The basic idea is that you're targeting people who are already interested in the topic (they're reading the comments because they already care about it) so they're much more likely to click your link. Plus you might also catch the eye of the blog's author, who could contact you and potentially even write a post about your product. You're basically trying to tap into that blog's audience.
There are no "bebefits" of blog commenting to drop a link. It's an idiotic spam-tactic. However, Rand Fishkin did a great Whiteboard Friday on how to build relationships through blog commenting (NO, for the lazy people who can't be bothered watching the video (or reading the transcript): this is NOT Rand Fishkin recommending spamming blogs): Comment Marketing as an Inbound Tactic - Whiteboard Friday
Blog commenting isn't blog spam. Spam is spam, and if we're honest, a lot of marketing (regardless of the spin you put on it) is spam. Not many people want to see advertisements, ever. The links in comments help create a larger network of connecting nodes to and from your site. It's always a good idea to contribute to other sites to bring in visitors who might be interested in your comment/post, or by building a relationship with the owner of the site you're commenting on. Also, if it's on the homepage, you get a good SEO boost if the PR is high
This! Blog commenting for the sake is SEO is nothing more than spam. You can stick lipstick on a pig, but you still got a pig Nigel
Randomly blasting directions with comments won't bring you very far. But if you actually take time to read the article or just take a glance and try to somewhat summarize it and drop a link to your site. You have to be creative to gain the interest of the blog's readers, that's why the content of the comment is really important, rather than the aspect of ranking the site. Sincerely, mindwire
Hi, Blog commenting is the best way to get backlinks when we do in a right way i.e., without spamming commenting should be based on your niche.