Final answer:
Passing an argument by reference is useful when modifying a large data structure directly, such as sorting an original array in-place without creating a copy, thus saving memory and processing time.
Step-by-step explanation:
An example where an argument should be passed by reference is when you are updating or modifying a large data structure such as a large object or array in a function and you want those changes to reflect in the original object without having to return it explicitly. By passing the argument by reference, the function can directly alter the original data structure. A typical scenario might involve a sorting algorithm, where the original array needs to be reordered. Instead of creating a copy of the array, passing it by reference allows the function to sort the original array in-place, saving memory and processing time.