The balance between unity and variety in music is a technical/artistic challenge that composers have been grappling with for probably as long as they have been writing music. It seems to become a more urgent issue, however, when you head into the nineteenth century with the development of the idea that works should be individual – that they should have recognisable identities that distinguish them from all other works. There are all sorts of reasons for this aesthetic shift, but changes in listening habits are indicative. When the orchestra is the background music for a duchess’s card game, symphonies can be more generic, but when the lights are lowered to put everyone’s attention on the orchestra, the music needs to do more to distinguish itself.
Those of us writing a cappella arrangements of popular songs from the last century might be working on a more modest scale than the nineteenth-century symphonists, but we face similar artistic and technical challenges. Specifically, there is the question about how much to repeat stuff. It’s a simple question, but the answer is always interestingly non-obvious, because of the effect that repetition has on a piece’s sense of identity.