Another way to look at it is: If X is a really good website in its category, and Google consistently puts X above other websites in search results for that category, the user experience on Google is better with X than without X. This will lead to more users using Google more, and contribute to all the effects that make Google money.

So even if Rap Genius is not bringing revenue to Google, Google certainly can derive value from Rap Genius that contributes to revenue. However, since the effects are indirect, it's not easy (though not impossible) to put a monetary value on this relationship.

Another way to look at it is if Google detonates start ups from VCs they co-invest with, will their own internal venture arm get squeezed out of some deals?

And if implicit or explicit risk protection (and back channeled information about potential upcoming risks) comes as a fringe benefit of direct investment, doesn't that "free insurance" allow Google Ventures to guarantee themselves more favorable investing terms?