There are a lot of reason why people comment on blogs. The first and most obvious reason is to get something out of it. People want to advertise their site by comment on your blog. They get to leave their site’s URL on the comment box.
The second reason. They want to be heard. They read your article and want to voice their opinion. They want to ad to the article. They either disagree or agree and want you or your readers to know it.
The third reason. Other bloggers want to network with you. They figure that if they put something useful on your comment section, you might check them out. Or someone reading your blog might check them out. Commenting also give them an opportunity to announce their presence in the blogosphere. By leaving their URL, they can announce to the world the existence of their blog. Commenting also makes blogs more visible. It helps with technorati ranking too.
Generally speaking, make money blogs have commentators that are looking to network more then anything. Entertainment sites have commentators that just want to voice their opinion. Tutorial sites have commentators that are seeking answers.
Having a lot of comments on your blog is always a good thing. Make sure you have akismet intact and make sure you write articles that welcome comments.