Is it to remind yourself and your team to implement the class correctly?
I don't fully get the use of an abstract class like this:

```
class RectangularRoom(object):
    def __init__(self, width, height):
        raise NotImplementedError

    def cleanTileAtPosition(self, pos):
        raise NotImplementedError

    def isTileCleaned(self, m, n):
        raise NotImplementedError

```