How should I use the Optional type hint?

I'm trying to understand how to use the Optional type hint. From PEP-484, I know I can use Optional for def test(a: int = None) either as def test(a: Union[int, None]) or def test(a: Optional[int]).

But how about following examples?

def test(a : dict = None):
    #print(a) ==> {'a': 1234}
    #or
    #print(a) ==> None

def test(a : list = None):
    #print(a) ==> [1,2,3,4, 'a', 'b']
    #or
    #print(a) ==> None

If Optional[type] seems to mean the same thing as Union[type, None], why should I use Optional[] at all?