Really Nice Build there! But it sure as heck dosen't look like any that have to my House. They all look like they've run the Baja 1K - LOT's Of Dirt Roads here.
I think the question was WHY a Potvin style Front Mount instead of the most often seen Top Mounted Belt Drive setup, not what it did. As Tony said the top mount affords the chance to vary pressure by changing the drive ratio where the Potvin has a fixed psi vs rpm. The Potvin'd have 2 advantages over a Belt Driven unit. One being a Direct Drive there'd be less parasitic loss through the drive gears and belt and the other being a lower profile making for less aerodynamic drag in use.
Enjoyed watchin' this one grow and the result was worth the wait.
What I found interesting was an almost parallel project on Full Custom Garage on MAVTV.