Backlinks are extremely important for SEO. And one method of building them that has long been popular is blog commenting. Many people still subscribe to the theory that if you just write heaps and heaps of them on countless blogs in your niche then the combined effect will be good for your search engine rankings. This is why there is so much blog comment spam out there, even today.
I still see it on some of my bogs, even from SEO companies. That bothers me greatly because they must be using it to promote their clients' sites as well. Think of all those bad links that they're building that will eventually be penalized by Google. To undo the damage, the site owner will have to get them removed. What a nightmare!
So, obviously, commenting should not be seen as a high quantity strategy, but one focusing on quality instead. That is, you should use them to get on the radar of blog owners as well as other bloggers who are adding to these threads.
If they like your input they will often comment on your blog -- and just as you have done, they will do that thoughtfully. Obviously that's a good thing, because we all love getting good quality comments. (And you should reply to them promptly, too. Readers who see at least two comments are more likely to add their own thoughts. And so the process continues.)
And the more comments you have, the more content is there to be found. That's another benefit for SEO. When looking though my stats I've found that those blog posts that have the most reader contributions tend to rank higher than those with few or none at all.
The other benefit you get from writing good comments on someone's blog is that the owner might even include a link back to yours in a post. That is even more helpful SEO-wise, obviously.