The potential energy of an object is determined by its mass, its height above the ground, and the acceleration due to gravity. In the case of a roller coaster car, the mass of the car and the acceleration due to gravity are constant. This means that the only way to increase the potential energy of the car is to increase its height above the ground.
By making the hills on a roller coaster high enough, the designers can ensure that the car will have enough potential energy to reach the end of the ride, even if it is empty. In addition, the hills can be designed to give the car a thrilling ride by providing just enough potential energy to make it over the top of each hill, but not so much that it flies off the track.
The design of a roller coaster is a complex process that involves many factors, including the height of the hills, the length of the track, and the speed of the car. By carefully considering all of these factors, the designers can create a roller coaster that is both safe and exciting.