Y-Combinator maintains this platform ( in my own opinion ) for two reasons 1) it's a neat place for likeminded people to share and discuss interesting topics

Y-Combinator maintains this platform ( in my own opinion ) for two reasons 1) it's a neat place for likeminded people to share and discuss interesting topics 2) this is a byproduct of 1. by gaining popularity from 1. they gain more exposure to a lot of smart people, which turns out great when they post job ads, or new companies reveals, as well as a lot of people apply to y-combinator, which in turn means they get to be very picky in who they invest. But HN doesn't have a business model per se. same with Reddit communities.