The first use case imaginable is a smart asset, or token, built on top of a currently existing blockchain.
This is literally building your own currency on top of another currency.
All that needs to be done is to ensure that a user who wants to spend money has the funds, has been authenticated, and is not double spending.
Authentication and double spending protocols are both built into Ethereum , so the only piece of logic that the smart contract needs to handle is checking for funds.
Thus, it only requires two main features to be functional: a storage structure for associating addresses with assets, and a send() function to transfer assets.
An example of the send() function is shown below, written in Solidity, a popular contract oriented, high level EVM language for implementing smart contracts.
This demonstrates that building an app on top of the Ethereum blockchain allows you to tap into the currently existing blockchain protocol and procedures.
All that the user needs to provide is the basic, secure logic to bring their desired blockchain use case to life.
It should be noted now that building functional smart contracts and building secure ones are two completely different things.
Securing smart contracts is incredibly tough, as you may have figured out from recent news about smart contract hacks.