I believe the idea is that if you stored the result instead, you would have to wait for the promise to resolve the first time. If you made two requests in quick succession, the second request wouldn't see anything in the result map for the first one (as it hasn't resolved yet) and would then need to make its own new request.

If you store the promise, then not only does the second attempt know that it doesn't need to make a new request, but it can also await on the promise it finds in the map directly.