For me, these historical questions are moot. It makes as much sense to ask “Why break the mass into different sections?” or “Why collect different dances together?” One simple answer: there are many ways to articulate the passage of time, and the passage of time remains one of the most fascinating challenges we grapple with on a daily basis. So why not take advantage of all the ways in which time can be projected through the course of a composition?
Here’s the fun part of multi-movement forms for me: the possibility of having several beginnings and several endings. Beginnings and endings are spectacular moments in life and art, all of them unique and yet all closely related. When I write a piece in four movements, for example, I get to think about four ways of beginning and four ways of ending, all of which can complement or contradict one another.
The parallel perq of writing a single-movement piece is coming up with a variety of transitions from one section to another. Sometimes transitions can be abrupt – now I’m here, now I’m not – and sometimes they can be protracted affairs that take on the weight of independent sections. Between these two extremes, the composer has the opportunity to create the connection that most precisely matches the need.
So should music stick to single-movement forms or multi-movement forms?
As far as I’m concerned, music does a great job of sticking to everything.