What are they going to do with the money? How good can they make their website?
There are no doubt useful companies out there for Pinterest to buy, no different than there have been for Facebook, Twitter, Google, et al. over the years of their various stages of maturity.
At Pinterest's size, and given their long-term ambition to be a large, successful business... it's for all the non-obvious reasons you can't think of that a billion dollar cushion might be useful. Always raise money when you don't need it; raising it when you need it, is the worst time, you'll go begging on your knees or at a minimum will be in the weaker negotiating position.