Say you have a function which expects objects of type Foo as an input and which returns objects of type Baz. One day, the function is improved by also accepting the type Bar, i.e. Foo|Bar. So Foo isn't needed for the computation, because Bar is also accepted.

Or you have a function which expects objects of type String as an input. But then you realize that in your case, null values can be handled just like empty strings. So the input type can be relaxed to String|Null.