Most paywalled submissions are to major sites that no doubt many people here subscribe to. I'm sure that there are a lot of Economist subscribers. Same for the New York Times, the Wall Street Journal, and the Washington Post. Probably a good number of Bloomberg and Forbes subscribers, too.
Enough of those people typically read and comment on the article, often quoting the parts that they reply to, to make the comments interesting and informative to those of us who have not read the article.
Also, HN comments often go off on a tangent, and those tangents are often the best part of the discussion.
If a person can't read the article and the comments aren't useful without having read the article--my recommendation is to move on to another story. HN has sufficient volume that there is no way most people have time to read more than a fraction of it anyway.
Finally, if you notice that there are a lot of submissions that you really do want to read coming from the same paywalled source, that's a good indication that maybe that is a site you should subscribe to, either directly or as part of some aggregator like Apple's News+ service.