Personally, I find reading the comments section gives a good taste of the thesis of the article, and sometimes the thoughts of the commenters are way more insightful than the article itself.
There are other instances when I get the gist from the headline and don't care about the story, but want to read what others have to say (for example, the current front page story about New Zealand banning foreigners from buying new houses in the country. Headline says it all but there must be something interesting to talk about if a story like that garners 444 comments)
There are two reasons I visit the HN front page instead of getting all my news off comment-free aggregators like Google News or more specialized equivalents.
Reason #1 is that often HN surfaces articles on topics that I wouldn't see anywhere else without a lot of digging through noise. Those I usually click straight through to the article and skip the comments.
Reason #2 is that sometimes I've already read about a topic elsewhere and feel strongly about it and want to express myself or hear how the larger community is thinking about it. Because someone is wrong on the internet! Those I dive into the comments first.
Occasionally an article will satisfy both reasons, and then I'll do both but the order of operations depends on how well informed I already feel about the topic. The better informed, the more likely to start with the comments.
Things I continue reading:
Perfectly spaced 5/6 lines of paragraphs, explaining without sounding smart :)