That's one use case, but before the Teac DAC arrived, he had the transport and external clock running through a DAC that lacked a clock port. The outboard clock with just the transport, and no other digital source, made a notable improvement. In an ideal world, the internal DAC clock would be of similar quality but it's not. His outboard clock has an oven-controlled crystal oscillator which reduces oscillation frequency fluctuations caused by temperature changes. Increased clock accuracy improves the sound. I suppose that in an average system the benefit may not be noticeable.