Writing ability and what is written shows thought processes, goals, priorities and other areas important to BYU and other schools. I have talked with and attended multiple schools and all have writng at the top of their requriements. It isn't just writing ability, but what is written. The questions that you are answering/ topics you are given are chosen carefully, so they can see your priorities, thought processes, etc.

To BYU recommendations from semionary teachers and others are very important, because they are looking for people capable of success in a very unique environment. An environment most people, including most Mormons, will be not only uncomfortable in, but unable to succeed in.