The advantage is that it goes from being parallel to the ground to being perpendicular to it without any jerks that could bump the ball off.
I am not sure if you say this sketch from the other picture thread:
Garage rails weigh a lot... hence those cheese holes (all those cheese holes make convenient places to put surgical tubing!)
EDIT: Also, it was much easier to make one weld in the garage rail than to cut carefully, measure perfectly and make multiple. The rollers slide better on the preformed shape too.