The point is not computational time; the point is that one could simply call an existing library function rather than hand-coding the loop oneself and risking making an error (a fencepost error, for example).
I can understand that you'd want to manually check what's happening. For example taking the mean over the rows of a 2D array using numpy's mean function and aren't really sure whether axis=0 or axis=1 refers to the rows.
But you'd only have to figure it out once and then learn to trust numpy, instead of rolling your own version every time.
But you'd only have to figure it out once and then learn to trust numpy, instead of rolling your own version every time.