Rollout is important for cars that use foam tires (or any tire that may significantly change its diameter.) It is used in the same way that gear ratio is for other cars, but it takes into account the tire diameter as well, since tire diameter varies for cars that use foam tires.
For example, let's say you're driving a 1/12th scale RC pan car with brand new foam tires you just mounted, and you look over, and the racer next to you has the same pinion and spur, but they just finished sanding down their tires, so they're 5mm narrower. How do you think this affects the car?
Larger tires have the same impact on the final drive ratio as using a larger pinion - larger tires produce higher top speed and less acceleration (and they also change your ride height!), so even though you may have the same gear ratio is that other driver, you have a lower effective gear ratio.
To account for this we use a number called rollout which is a true measurement of the effective gear ratio.
If your tires are always the same size (for rubber tires), FDR is all you need, but if your tires can change sizes (usually only significant for foam tires) you need to use rollout to understand your effective gear ratio.
Rollout definition: how far the car moves for a single rotation of the motor (or pinion.)
rollout = tire circumference / FDR or, for pan cars with no transmission:
rollout = tire diameter * pi / (spur / pinion) or
rollout = tire diameter * 3.1415 / (spur / pinion)
A higher rollout number means the car moves farther every time the motor turns which would have the same effect as a lower FDR or larger pinion.
Again, let's say the racer next to you is going crazy fast, so you ask them what their rollout is. When they tell you, then what? What pinion should you use? Backing the formula around, you get:
pinion = (spur * rollout) / (tire diameter * 3.1415)
Learn more about gear ratios: Rollout, Final Drive Ratio, and Transmission Ratio.