After all, there's an upside for him. He gets more content on that post. That helps his SEO. It also means that there'll probably be still more comments following. That's why he's quite likely to comment on your blog as a thank you. And just as he was rewarded, so will you be ...
And you don't just get clicks from the blogger himself. If you do this on blogs with decent traffic early on -- preferably before anyone else has -- then you'll get some clicks from readers, too. Actually, if you regularly add your thoughts to many blogs in your niche there's no reason that you can't bring in an extra five or ten clicks a day that you simply wouldn't have been getting otherwise. That's thousands of extra hits a year. Not to be sneezed at.
Commenting also delivers backlinks that can help with SEO, although this effect has become a lot less powerful in recent years. Because of this benefit from "dofollow" blogs, people still spend an awful lot of time looking for them. This is a silly obsession in my opinion. If you use this technique on scores of blogs some of them are sure to be "dofollow" anyway. In any case it's important to have quite a few nofollow links in your profile. Too few and it will look suspicious to Google.
There's another benefit that is less obvious: By commenting often, you'll be amassing lots of extra content to use for your own blog at a later date. That's why you should get in the habit of cutting and pasting them into files after you've submitted them. (Needless to say you do have to rewrite this material before publishing. That will make it unique content.)
So, blog commenting has a lot to recommend it. Which is why you should do it often on blogs in your niche. But the only way this works is if it's of a good quality. Otherwise you'll just be spamming. And that's the last thing you want to do for a whole bunch of reasons.