One of a couple of reasons. It's not likely to be as well developed in a review from 81 but an important part of modern high performance sailing is gust and wave response. That is, when you get hit by a gust or a wave, how does your rig and sail flex in response to help you manage. Generally, having the top of your sail twisting off can help with designing for gust response.
Back in the days of square sails, standard practice was that sails higher up on the masts were trimmed looser , in order to give the helmsman time to react when the wind direction changed in a gust (particularly when sailing close-hauled, of course).
