Python Iterators: Use of __iter__ method in a class [duplicate]

I have been reading about Iterators in the python 2.7.x Documentation The given sample program in the link defines a __iter__ method to make the class "iterable".

However, i cannot find any use of __iter__ method defined in the class, i.e. to say- Even if i don't define __iter__ , i can still iterate upon the object of the class.

Code 1 # From the documentation

class Reverse:
    """Iterator for looping over a sequence backwards."""
    def __init__(self, data):
        self.data = data
        self.index = len(data)
    def __iter__(self):
        return self
    def next(self):
        if self.index == 0:
            raise StopIteration
        self.index = self.index - 1
        return self.data[self.index]

Code 2 # Except for defining the iter method it's the same.

class ReverseMod:
    """Iterator for looping over a sequence backwards."""
    def __init__(self, data):
        self.data = data
        self.index = len(data)
    # Missing __iter__()
    def next(self):
        if self.index == 0:
            raise StopIteration
        self.index = self.index - 1
        return self.data[self.index]

Output:

rev = Reverse('spam')
revMod = ReverseMod('spam')

rev.next() #m
rev.next() #a
rev.next() #p
...

revMod.next() #m
revMod.next() #a
revMod.next() #p
...

Objects from class Reverse and ReverseMod behave the same way. Only thing is when i do a iter(revMod) it says

TypeError: iteration over non-sequence

Why do i even need to concern myself about doing iter(revMod)? Why do i need to care defining __iter__ at all. Seems like i can get the functionalities of doing a .next() without defining __iter__ at all.