The idea is to hit different energy systems.

In this case the ATP-CTP and Anaerobic energy systems. They cannot resynthesize ADP into ATP (to release energy in the working muscles) for longer than about 15 seconds, so if the incline/distance is taking you longer than that to cover then you're at a high enough intensity to be breaking the thresholds for those energy systems to become stressed.

Simply put, you can only work at max intensity/effort for about 15 seconds...