Class definition within a function or another class

I've seen in some scripts definitions of classes within other classes or functions like this:

class Grok(object):

    class Foo(object):
        ...

    ...

What is the reason for this practice? Is it just to not allow direct instantiation of the internally defined class, and is this really a good practice or should I avoid its use at any cost?

class Circle:
        class DrawingAPIOne:
                '''Implementation-specific abstraction'''
                def drawCircle(self, x, y, radius):
                        print("API 1 is drawing a circle at ({}, {}) with radius {}".format(x, y, radius))
        class DrawingAPITwo:
                '''Implementation-specific abstraction'''
                def drawCircle(self, x, y, radius):
                        print("API 2 is drawing a circle at ({}, {}) with radius {}".format(x, y, radius))

        def __init__(self, x, y, radius):
                '''Implementation-independent abstraction; Initialize the necessary attributes'''
                self._x = x
                self._y = y
                self._radius = radius
        def drawWithAPIOne(self):
                '''Implementation-specific abstraction'''
                objectOfAPIone = self.DrawingAPIOne()
                objectOfAPIone.drawCircle(self._x, self._y, self._radius)
        def drawWithAPITwo(self):
                '''Implementation-specific abstraction'''
                objectOfAPItwo = self.DrawingAPITwo()
                objectOfAPItwo.drawCircle(self._x, self._y, self._radius)
        def scale(self, percent):
                '''Implementation-independent abstraction'''
                self._radius *= percent

I saw this code on the site: link