davearonson said almost 7 years ago on Routing Partials :

Just wondering about the use of .map there.  Does the mapper actually somehow use the result of each call, so that it becomes important to use .map, or is the important part more of a side-effect, so that .each would do?  If the latter, I think that would be clearer as it wouldn't raise this question, in the minds of those of us who tend to overthink things.  :-)