It's what spins out a phrase, makes the listener hungry for what's next, and, ultimately helps give you the coherence of the music. It's an art that can take a lifetime to master and for me defines a great musician. Needless to say, the Warland Singers have such a line that many of these tracks, despite their length, seem to be taken in one Homeric breath. The line is also an art that belongs to a conductor. One way to shape it comes through dynamics - crescendo and diminuendo.
Most choirs can get louder.
Getting softer and softer dynamics in general really test them. One very seldom hears a piano or pianissimo without flab or breathiness. It's the difference between mumbling and an intense whisper. This is also true of orchestras, by the way. Furthermore, many conductors today don't really understand the importance of falling back to build again and don't insist on it from their players - one reason to me why so many current performances go nowhere.