This is explained pretty well in the types module description. It shows you that types.SimpleNamespace is roughly equivalent to this:

class SimpleNamespace:
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

    def __repr__(self):
        keys = sorted(self.__dict__)
        items = ("{}={!r}".format(k, self.__dict__[k]) for k in keys)
        return "{}({})".format(type(self).__name__, ", ".join(items))

    def __eq__(self, other):
        return self.__dict__ == other.__dict__

This provides the following advantages over an empty class:

1. It allows you to initialize attributes while constructing the object: sn = SimpleNamespace(a=1, b=2)
2. It provides a readable repr(): eval(repr(sn)) == sn
3. It overrides the default comparison. Instead of comparing by id(), it compares attribute values instead.

SimpleNamespace is basically just a nice facade on top of a dictionary. It allows you to use properties instead of index keys. This is nice as it is super flexible and easy to manipulate.

The downside of that flexibility is that it doesn't provide any structure. There is nothing to stop someone from calling SimpleNamespace(x=ax, y=ay) (and del a.z at some point later). If this instance gets passed to your function, the exception occurs when you try to access the field.

In contrast, namedtuple lets you create a structured type. The type will have a name and it will know what fields it is supposed to have. You won't be able to make an instance without each of those field and they can't be removed later. Additionally, the instance is immutable, so you will know that the value in a.x will always be the same.

It's up to you to decide if you need the flexibility that SimpleNamespace gives you, or if you prefer to have the structure and guarantees provided by namedtuple.