An inclined plane is, essentially, a ramp. Inclined planes are useful for a wide range of physics experiments, and they can demonstrate basic physics principles about gravity and angular motion.
Some of the earliest experiments about gravity were conducted using inclined planes. One of the curious effects of them is that objects that roll down them have the same speed as objects dropped from the same height, but they take a longer time to reach their ultimate speed.
Ancient cultures used inclined planes for a variety of experiments as well, and they are one of the six classic "simple machines" of the Renaissance period. Without inclined planes, some of the simple rules that govern classical gravitational theory may not have been discovered until later.
Inclined planes also fill a number of practical roles. They were invaluable tools for lifting heavy items since they allow sideways motion to be converted into vertical motion, and many classic structures were built using inclined planes. Their simplicity also makes them popular; simply laying down a flat surface is enough to form an inclined plane, and they can be moved easily. While humans generally call them ramps, inclined planes are still in use in modern buildings.