Take an eCommerce system for instance, you want to open up complex shipping or discount calculations to your end users. A great way to to do this is to allow th

Take an eCommerce system for instance, you want to open up complex shipping or discount calculations to your end users. A great way to to do this is to allow them to write a function that takes in known objects and returns a result. Say it takes in a cart/line_items and returns back a calculated amount. Obviously its a bit more complex then that, but it is a great way to expose some functionality without exposing the entire system.