What was the main use for airplanes in World War I?

Initially used for surveillance, to take photos of enemy trenches so as to be able to better plan attacks on the ground.

Then used to attack other planes in "dogfights" (using machine guns mounted on the front of a plane with a specially timed propellor, to let the bullets out between the rotations)

Finally used to drop shells (not just on the enemy army, but their civillian areas as well) and fire machine guns from