Зачем нужен метод __next__

class Counter:
    def __init__(self, low, high):
        self.current = low
        self.high = high

    def __iter__(self):
        return self

    def __next__(self):
        if self.current > self.high:
            raise StopIteration
        else:
            self.current += 1
            return self.current - 1

# Использование
counter = Counter(1, 3)
for num in counter:
    print(num)  # Выведет 1, 2, 3