Advantages of UserDict class?

What are advantages of using UserDict class?

I mean, what I really get if instead of

class MyClass(object):
    def __init__(self):
        self.a = 0
        self.b = 0
...
m = MyClass()
m.a = 5
m.b = 7

I will write the following:

class MyClass(UserDict):
    def __init__(self):
        UserDict.__init__(self)
        self["a"] = 0
        self["b"] = 0
...
m = MyClass()
m["a"] = 5
m["b"] = 7

Edit: If I understand right I can add new fields to an object in a runtime in both cases?

m.c = "Cool"

and

m["c"] = "Cool"