How about this - adding ten cog allowed them to keep top gear ratio unchanged (?) with the smaller chainring and that means now you get lower gear with the same sprocket that got the ten cog added to it. It widens the gear range. I just made that up for whats it worth. If true, that would sell it to racers.
Serious hobby cyclists would buy it to get rid of cables and have clean looking bike as well as efficient shifting, less serious ones (my case) to make shifting less complicated - I rarely bother to downshift after changing to smaller chainring to compensate for big step in gear ration (but my old bike doesn't have such big difference between chainrings). Also if I did that with shift levers on downtube, I'd lose even more speed, so.