I would imagine dividing the column in to ten segments would leave them rather small and difficult to hit accurately, on screen keyboards for instance require a

I would imagine dividing the column in to ten segments would leave them rather small and difficult to hit accurately, on screen keyboards for instance require a certain amount of prediction to guide missed clicks; a fader lets you dial it in. If you've ever played a stringed instrument (fretless that is), you will know that it's easier to miss your note by a little and then roll your finger to adjust than it is to simply get the note first try without some kind of tactile feedback like a fret, or in the app's case, a physical button.