It is used when you do not know how many contents of the list are variable until you hit it.
list = ["a", "b", "c", "d", "e"] print list ['a', 'b', 'c', 'd', 'e']
print list a print list c
print list Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: list index out of range
print list[-1] e
print list[-2] d
If you know it, there is nothing you can do, but if you don't know it, it will be a powerful skill.