When I'm using music as an accompaniment to something I'm doing - in this case writing an essay - I don't want it to get "too" interesting all of a sudden.  The idea of taking a theme and slowly working it, gently twisting and developing it into new shapes and sounds makes a lot of sense to me.  It's different enough to allow the brain to notice the change, but not too different that it has to devote extra resource to processing it.