I think it’s used to check, if an object is initialized, e.g. if it’s optional such as a transformation.
You would often see Python code as:
def __init__(self, transform=None): # initialize transform as None
    self.transform = transform

def __getitem__(self, x):
    if self.transform:
        x = self.transform(x)

and could use it with any other Python object as well.