Real question--- why does one of the GitHub co-founders need to raise 17M for a venture? I'm certain they could fund it themselves. Is this more or less a marketing play than anything else?
So that when the venture inevitably fails, VCs are the ones that lose money and the founders get a nice exit.
Why do rich people raise money when they already have money?
1) because they can
2) it's their money, not company money, and again, why would you risk your own money when someone else wants to risk their money?