A little variety makes all the difference.

I believe part of the reason hymn arrangements are so effective in worship services is that when we hear the same thing over and over again, we begin to tune it out. It is expected, we know what is coming next so our brains turn to other thoughts. When the ward choir sings, a special arrangement gives our ear a chance to focus on a slightly different rhythm, or harmony. It makes us pay attention and to focus more on the message.