It is of value to be able to describe a type constraint on boolean true or false instead of all Booleans. Without this, certain variations cannot be described with the type system - e.g. "Give me a number or true" and additional logic is required.
The implementation could add a parameter to the boolean, but since there are only two possibilities, this is better modeled as subclasses.
(Given 2 points because of possible unit test work volume)