Understanding __get__ and __set__ and Python descriptors

I am trying to understand what Python's descriptors are and what they are useful for. I understand how they work, but here are my doubts. Consider the following code:

class Celsius(object):
    def __init__(self, value=0.0):
        self.value = float(value)
    def __get__(self, instance, owner):
        return self.value
    def __set__(self, instance, value):
        self.value = float(value)


class Temperature(object):
    celsius = Celsius()

1. Why do I need the descriptor class?

2. What is instance and owner here? (in __get__). What is the purpose of these parameters?

3. How would I call/use this example?