On a more serious note, Quora users have a better profile for advertisers because they are likely to be in the top brackets for income and education and unlike an English Q&A site the topics on Quora are limitless. You could target ads at 20-40 year females that have a PhD and live in New York very easily.
Also from the looks of it the decision might not purely be for financial gains. Adam D'Angelo is apparently one of the smartest people around (as referenced by Mark Zuckerberg in some interview) and also independently wealthy so keeping yourself in his good graces might lead to future positive outcomes not entirely related to Quora.
Also Tiger Global is run by a youngish hedge fund manager born into old school money family and he probably pals around in the same circles as Zuckerberg, Adam D'Angelo, Charlie Cheever, other young SV entrepreneurs with FU money and this seems a lot like friends helping friends (this is just a hypothesis which might not be true).
It can also be that the investors know something compelling that we don't know or are hoping that Quora will be the next big social thing after Facebook / Pinterest / Twitter / etc.