Without HN, the cofounder of a mobile startup doesn't know of any hobbyist iOS developers in Minnesota who might be a good addition to his team, because the hobbyist developer never sees stories about failed iOS apps as something worth writing about and even if he thought it was worth writing about without HN his story would never get in front of the right audience.

To put it another way, a startup in San Francisco doesn't need to go to the Midwest to find iOS developers. Just to find the right one.