Sounds like 2 genius ways for Google to extract more revenue from their core service.

One, they'll start seeing increased sales from companies who can now spend money on Adwords they don't currently have in the bank. This revenue is almost 100% new since the capital was not available before. Although, it's possible some companies were just using other credit cards to finance their ad buys.

Two, Google would start collecting interest. This doesn't sounds cheap if you consider the risk of default and actually collecting on those default. But they could probably sell those loans to collection agencies and still make a profit.