In the case of a callback, only at at runtime will you know which function to call
Discussion
So for example you have a function to add money to a bank account and a function to subtract money from a bank account and you don't know which the user will choose to do when they use the application?
Yeah it's stuff like that. Let's say you are a caller of a function that does a withdrawal based on the result of a validation. The caller wants the function to use withdrawal method A, and provides the method. There is more than one withdrawal method, so that's how the caller is telling it which one to use