Obviously the non-financial reinvestment in the startup community & ecosystem, means they're making a positive contribution that should see more / better startups coming to the fore and when they need funding, USV have already "bought" the good karma by giving back to the community. At that point (if USV decides to make an investment), they know that they helped shaped the startup (via the community) and if they exit from the investment, they will have a financial gain.
For me, this represents the perfect blend of altruism & profitability; two motivations that aren't necessarily exclusive and can work together. If your contribution to the community / ecosystem is sincere, then there's no reason why you shouldn't also potentially reap the rewards of that investment down the line. In USV's case, they are also helping a bunch of other startups that are signed up by other investors, who thus cashes in USV's altruism.
This balance resonates so much with me and is one of the reasons why I've decided to create The Rockstar Business Workshops. I love teaching and that's my top reason for setting up these workshops. But I also want to help nurture local entrepreneurs. And if that means that I may (or may not) be part of their future projects (as an advisor / mentor / investor / co-founder), then that is just a bonus.