A volume pedal and expression pedal are not quite the same thing, which is why they have separate midi commands for each. The idea is that you set the volume of a patch at a preset level using the volume control (or CC #7) and then use the expression pedal (CC#11) in order to vary the volume within that volume range without exceeding it. It's especially used by organists but it's also useful with brass patches for doing fade-in horn stabs.