When would you use reduce() instead of sum()?

Question

I began learning functional programming recently, and came up with this example when attempting to calculate my quiz average for a class.

The example I came up with

Answer 1

In place of sum? Never.

But a reduce call would be the way to go when aggregating by a custom method.

For instance product could be defined as:

product = lambda iterable: reduce(operator.mul, iterable)

Also sum is implemented in C.