This often comes up when writing a function which returns a wrapper over a generic type (like Option<T>). If your Option type is T | null, then there's no way to distinguish between a null returned by the function or a null that is part of T.

As a concrete example, consider a map with a method get(key: K) -> Option<V>. How do you tell the difference between a missing key and a key which contains `null` as a value?