Blog comments complete the interactivity of every single blog post. Without their presence, it feels like a storyteller is sharing a story to an empty room, boring and devoid of any interesting passage. Comments whether affirmative or antagonistic provide a good avenue to expand the idea behind a story, an opportunity to explore its strengths and weaknesses and generally learn from it. Maybe reading a blog post is good enough, but adding our comment also has its benefits.
We get noticed
By posting a comment that effectively proves (or disproves) the point written on the blog or raise the level of discussion, we start to get noticed by the others. Of course we can't do it all in one go, but somehow we start to establish reputation online. People could start clicking on our profile links and explore our blogs/websites.
We present our unique idea
The whole concept of of blogging is to present a personal idea. But unlike corporate write ups that often disallow feedback to be posted, blogs are casual enough to ignore this interactivity. If Twitter asks what are we doing, blogs ask what do we think of the writeup. That's a great medium to react and leads us closer to authors.