Not everything has to have an explicit purpose beyond “this is a good and valuable thing”.

Why do I take in parcels for my neighbour if a courier knocks on my door? She doesn’t pay me. It wouldn’t cause me any harm if I didn’t. But it makes the place nicer to live, and I’ve become friends with her as a result.

She invited me to dinner recently and fed me delicious food, and we drank very good champagne. That was an unexpected bonus.

“To promote ycombinator” only works if there’s an audience worth promoting to. Building something great that brings people back day after day maybe has the result that it can also serve as a promotional tool - but that’s a bonus, not necessarily a purpose.

I’m not the person you asked the question of - but I think the purpose of ycombinator is to give relevant people a place to discuss things aligned with the ecosystem in which ycombinator operates, to help strengthen and champion that ecosystem. Does it have a payoff for ycombinator? Almost certainly. Was it created with that explicit purpose in mind? I doubt it. There are easier ways to make money.