The simple answer: To protect the lives of the drivers, to make it easier to move uphill. The more specific explanation belongs to the scientific and physical aspects.
Simply put, in science, if the driver has to drive in a straight line uphill for a long time, it will affect the vision, cause distraction, and easily cause danger. The curved road will stimulate the driver's concentration.
From a geographical perspective, mountains often have rugged terrain, so to βflattenβ into an inclined line from the foot of the mountain to the top is more difficult to design, and in some cases even more expensive than making a zigzag road.
From a physical perspective, according to the principle of the inclined plane: a straight slope will be dangerous both up and down. Making a zigzag road is to reduce the slope, otherwise in the downhill direction, reduce the speed for the downhill direction, and reduce the inertia for the uphill direction.