Compliant wheels are not intended for moving robots, but moving objects. Compliant wheels give way to allow rigid objects to be manipulated easily. A line of multiple wheels on a driven hex axle creates a very effective roller that ignores variations in the objects being driven. This makes for high-performance intakes and internal conveyors.

Different durometer ratings give each wheel unique characteristics. Green wheels (35A durometer) are the softest and most compliant around objects, while orange (40A), maroon (45A), blue (50A), and black (60A) wheels are increasingly firm. The increased firmness also allows the latter wheels to carry a higher RPM rating. Check out the downloads tab for a chart showing wheels and their maximum RPM. 
Please note maroon wheels are only intended for use in FTC. If more powerful motors are used to drive it the spokes may fail.