Also the free advertising for the brand of Y-combinator. With the reach HN has these days having your name out there right in the domain means that the on balance positive reputation of HN rubs off on your enterprise. It's basically a huge 'sponsored by' byline. This doesn't necessarily translate into a net positive for a company, but in this case I am willing to bet it does.

It's honestly a fairly transparent transaction, and much preferable to the opaque world of on-line targetted advertising. I like how my ad-blocker just sits here twiddling its thumbs.