TRE said:
For aerodynamics or to get more weight in car
Aero is a factor, but weight is by far the biggest factor IMO. Its all about being able to put the weight where you want it.
Last year I built a "block car" for my 5 year old to race in the sibling races. Well, it looked like a normal pinewood block, but really it was a thin pine frame with balsa on top. I was able to get over 3 oz of weight into the back of it. It took first place in siblings. In fact, other than our family cars, there was only one car in the entire pack that was faster.
Here it is on the right (obviously) ....
And a peak at the underbelly ....