As mentioned, it's a remnant ad network. As for why they're doing it, well, look in a mirror. Advertising has turned, as the saying goes, print dollars into online nickels. If you wish to have free online content, something has to pay the bills.
Honestly, ad devs are some of the savviest javascript developers out there. We have a lot of constraints and need to move fast and scrappy on a daily basis.
I won't be doing it forever however it's one of those things that holds my interest until i get in my car and go home for the day.
* I work for a very large company that own radio & publishing properties.
So the model for reddit and HN is to charge for access, and pass on to the articles clicked through to. However in that game, I would pay NYT a long time before I paid Reddit.
Reddit and hn are entirely different: they pay nobody to produce content, nor do they charge for access.
Production will be valuable - but the curation is the secret sauce.