Зачем нужна функция enumerate?

for i in range(0,len(A)):

for a in A:

from collections import deque
 
s = deque([-4, 2, -3, -5, 7, 10, -1]) 
 
def test1():    
    for i in range(len(s)):
        a = i, s[i]
 
def test2():
    for i, d in enumerate(s):
        a = i, d
 
def test3():
    count = 0
    for d in s:
        a = count, d
        count += 1

if __name__ == "__main__":
    s = deque([i for i in range(1000000)]) 
    s2 = [i for i in range(1000000)] 
    
    setup="from __main__ import test1, test2, test3, s, s2"
    print('test1:',  timeit.timeit('test1(s)', setup=setup,number=3))
    print('test2:',  timeit.timeit('test2(s)', setup=setup,number=3))
    print('test3 :', timeit.timeit('test3(s)', setup=setup,number=3))
    print('test1:',  timeit.timeit('test1(s2)', setup=setup,number=3))
    print('test2:',  timeit.timeit('test2(s2)', setup=setup,number=3))
    print('test3 :', timeit.timeit('test3(s2)', setup=setup,number=3))  
    """
    test1: 74.07750789824419
    test2: 0.4306260096345653
    test3 : 0.5972959676934408
    test1: 0.45880559326029413
    test2: 0.42931033045138633
    test3 : 0.5868087143778098
    """

test1: 41.4063298702
test2: 0.0832080841064
test3: 0.0352368354797

import this