It's a perfectly valid volume control, and is quite often used in mixers - however, in this case it's a further example of design with little or no understanding of what they were doing. The entire opamp stage with volume control isn't needed at all, it actually gives a negative gain of ten, and from there down to infinity, plus it's fed from a low impedance buffer - it could be more sensibly replaced with a simple pot and series resistor, saving an opamp and the bis components etc.