Зачем нужна функция enumerate?

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

for a in A:

for i, d in enumerate('data'):
    print(i, d)

>>> l = ['joe','lee','nick']
>>> for i in l:
    if i == 'joe': i = i.upper()
 
    
>>> l
['joe', 'lee', 'nick']
>>> for x,i in enumerate(l):
    if i == 'joe': l[x] = i.upper()
 
    
>>> l
['JOE', 'lee', 'nick']
>>>

for i in range(len(l)):
    if l[i] == 'joe': l[i] = l[i].upper()

s = 'data'
for i in range(len(s)):
    print(i, s[i])

import timeit
 
 
def test1():
    s = 'data'
    for i in range(len(s)):
        a = i, s[i]
 
 
def test2():
    s = 'data'
    for i, d in enumerate(s):
        a = i, d
 
 
print(
    timeit.timeit("test1()", setup="from __main__ import test1", number=10000000)
)
print(
    timeit.timeit("test2()", setup="from __main__ import test2", number=10000000)
)

>>> names = ['Bob', 'Alice', 'Guido']
>>> list(enumerate(names))
[(0, 'Bob'), (1, 'Alice'), (2, 'Guido')]
>>> list(enumerate(names,1))
[(1, 'Bob'), (2, 'Alice'), (3, 'Guido')]
>>>

>>> list((i,names[i]) for i in range(len(names)))
[(0, 'Bob'), (1, 'Alice'), (2, 'Guido')]
>>> list((i+1,names[i]) for i in range(len(names)))
[(1, 'Bob'), (2, 'Alice'), (3, 'Guido')]
>>>

for i, s in enumerate('ABCD', start=1):
    print(i, s, sep='-')

print(*enumerate('ABCD', start=1))

a = [x**3 + i*x**2 + x for i, x in enumerate(some_array_of_numbers)]
b = [y*i for i, y in enumerate(x for x in arr if x > 0)]

    for i, fi in enumerate(eq):
        if isinstance(fi, Equality):
            eq[i] = fi.lhs - fi.rhs