One use case that will for sure be filled with cryptocurrencies like Ethereum in the coming years is that of internet money.
Just like we had an era of web-first applications and shops, which then became mobile-first. Internet-first money is yet be fully realized. Money that is not tied to a physical entity per se, but at the same time can store and be a medium of exchange of value over the internet.
Money, Paypal credits, Credit are all good forms of value individual users or businesses use to transfer wealth between each other. This can be done via online too but the underlying mechanism is still tied to bank accounts and balance ledgers maintained by different banks. The internet only acts as a proxy for the transaction.
Internet money, on the other hand has the capability to sustain the transaction and it's underlying mechanism end-to-end within the internet. Imagine if API's wanted to exchange money and not users or businesses. Internet money would be the default way to do it.
By the looks of it, cryptocurrencies are the most poised to fill in this use-case.