The use case is for marketplaces (and maybe crowdfunding sites). An escrow service is a means to increase trust for both the buyer and seller sides. Funds are paid up front but held in escrow until goods are effectively exchanged or services rendered.

I realize that it is a major feature and requires more than just code, but I'd love to see another major player in this space that isn't PayPal. Balanced Payments and MangoPay are great but they aren't household names (that customers are likely to recognize) like Stripe is fast becoming.