The inclined plane is the simplest of the simple machines, but it's been instrumental in everything from (most likely) building the Pyramids to making canals navigable. It provides a way to move a heavy object while using far less force than hoisting it straight up.
While its use in human endeavors is taken almost for granted, the inclined plane may also have had an important role in the evolution of flight. Young birds running up steep ramps flap their wings before they can fly, and scientists studying the energetics of flight have found that birds use far less energy running and flapping up steep inclines angles than they do flying at similar angles. This suggests, at least to some researchers, that flapping up an inclined plane could have been a bridge between running and flying. Though these inclined planes weren't part of birds, they may have shaped the way their bodies adapted.