What do you mean by that!?! I think you mean to say "how blog commenting is good for search engine optimization"
They give you backlinks(if they are dofollow). Hence increase your rankings. Increase your Pagerank. Sometimes may also give you a small amount of direct traffic.
If you make useful comments on blogs and they allow you to place dofollow links, then you get a very good chance of having lot more traffic to your blog that what you have seen before.
The correct should be "spam comments" and it does not work as it used to be anymore. It was a good way for link building until a mass spam happens and Google had already fine tuned their algorithm handling this kind of link building by diluted it value to minimal or even zero in some case.
Blog comments can beneficial your website is they are "dofollow" and relevant to your website niche. Otherwise it would be considered as useless and a waste of times.
I guess you how blog commenting is beneficial in SEO, well in my experience it helps creating backlink and not just backlink a quality backlink if you d it well and do commenting only on the related sites with high page rank.
Blog commenting is an important tactic of search engine optimization. and if these are do-follow then they would be more useful for a website as they provide a quality backlinks to website.
Blog commenting helps. But it is of no use if you leave your comment on a blog which is highly spammed already with comments
For Blog Commenting most important part is .........if they DO follow ...........and dont use many times only thanks in comment.... post with proper conversation i believe in that......
blog commenting is very useful in getting good amount of traffic, if you doing blog commenting on do follow sites then its helpful in getting SERP