it's here to increase visibility of the ycombinator brand, it's startups, and the technologies related to what they do. discussions around it were going to happen anyway, so own those and shape.
create an ecosystem, and then mold / skim / ingest / bask in it.
it's the same reason why BigCorps try really, really hard to get ownership / mod control of subreddits, and/or having their marketing assets responding to discussions in their sub or elsewhere.
Good luck finding any objective distinction between HN, Reddit, WhatsApp, Signal, or email + listserv and "social media"!
Or finding any one of "social media's harms" that could not, in some world where Facebook, Instagram, and Tiktok did not exist, be delivered in just as socially harmful (and beneficial) a form by sufficiently-accessible versions of the apps and protocols you value and use every day.
I met someone recently whose primary addiction is Wikipedia.
For me, Signal and Hacker News are the most addictive pieces of software I still use.
Serial television (best delivered by WebTorrent and The Pirate Bay) is by far the most addictive, for me, so much so that I had to quit.
And you can definitely run successful social movements and political campaigns (for both very good and very bad things) over HN or WhatsApp/Signal, given sufficient adoption.