In general playing, the Part Select buttons are redundant, as it's more logical and intuitive to simply press the buttons next to the voices on screen - exactly the same result and, of course, the Part Select button indicators change with the on screen selections. IIRC, they're more of a 'leftover' from the days when we didn't have big screens.
They do have their uses, as has been demonstrated above, but for most players, they can be safely ignored.