One of the most insightful observations that I've ever heard regarding the purpose and/or use of high tempos in music came from a composer with whom I had the good fortune of studying a few years back. He was talking about the fact that Glenn Gould, being the iconoclast that he was, was much discussed for his vast departures from the standard tempos of well-known pieces. But he felt that his recordings were very helpful in understanding the forms or structures holding them together, because "a high tempo is like altitude; it allows you to see more of the terrain." I've never thought of tempo the same way since. Here's a short excerpt from the 1981 version of the Goldberg Variations: http://www.youtube.com/watch?v=zpx6hJZ0-9o&feature=related