To further explain, residual coffee goes stale, the oil in the coffee beans can go rancid and thus spoil your cup and if the retention varies you might miss you

To further explain, residual coffee goes stale, the oil in the coffee beans can go rancid and thus spoil your cup and if the retention varies you might miss your ratio or go overboard. This is in practice at most annoying, so you can indeed make good coffee without a zero retention grinder but for a 'coffee snob' the most important thing is consistency in the cup.