is it the general why, as in why the shell we do this if the first version of it works so well?
or is it a why do they do the same
thing?
for the latter, @Sky020 gave a good link with many explanations but if it is not enough, ask again
for the former, it is a way to present recursion, to show an alternative why. It could have been presented in many ways. This was the one chosen in the curriculum