The ability to raise money is itself a test - if you can't get third parties to invest then that's not a great sign. Google's judgement may be impaired by being so close to it.
They could easily self-fund, but then they wouldn't get that signal. Forcing a company to survive out of the nest will influence the decisions the stakeholders make, they can't relax knowing that Google will just fund things no matter what, they have to be focused on shipping something people want.
At least that would be my guess.
It's a clever way to try and prevent the lethargic energy that comes from being a big company along with the inability for large companies to innovate outside their narrow domain (often even within it).
