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?

No, it isn't "completely useless".

If you have a function that will normally return a string, but can sometimes fail due to reasons, you may wish to yield an error message in the latter case. So you're going to be returning a string, or a string.

It's not what the content of the data is; it's how you're supposed to interpret it. You have two cases, success and failure, and control will flow differently depending on that, not based strictly on the type of data at hand. We just model those cases in a type.