Correct me if I'm wrong but I actually thought that was the obvious reason to most applicants.
A $120k investment from YC doesn't provide that much runway. That kind of money doesn't give much wiggle room for founders to splurge on luxuries like Aeron chairs and fancy cappuccino machines. That amount is to allow a small team to live in near-poverty in the SF Bay area for ~6 months and maybe also buy some tech necessities like AWS EC2 time. Basically, provide some breathing room for founders to not worry about rent and put the focus on building their idea. What other reason to work 60+ hours a week in modest conditions unless it was the networking and interaction?