Tracks give cross country mobility, wheels give speed and fuel efficiency. Christie came up with the idea of taking the tracks off and using a chain to connect the rear drive wheel to a road wheel.
Having separate tracks tracks and wheels was a more complex solution but it meant the driver could switch between the two methods without leaving his seat. It was a common experimental idea of the time.
The most practical user of it was the Austrians with the Saurer RR7 who ended up building 140 of them that were taken into Wehrmacht service.