What are the main use cases of the Coinbase exchange?

Well, perhaps the best use case for Coinbase's exchange is Coinbase themselves. As a merchant processor they now have control over the buying and selling of bitcoins for their merchants. Rather than opting for other exchanges, they will be able to on-load/off-load coins on their exchange, more money for them.

In addition, I am confident, Coinbase will become the Stripe equivalent for all things Bitcoins. The exchange would provide APIs for startups in the US to work with and for those who wish to trade, being state side, the funding of the accounts and withdrawals would be super fast.