The guidestones themselves were probably put there by a person who believed a nuclear war or something would destroy civilization itself and bring humanity back to the stone age. They were built during the cold war afterall. It's the instructions on how to manage the earth and humanity after this, as well as an astronomical calendar. The time capsule might have stuff from the late 70's and early 1980 as a way to share the tech they had to the future post apocalypse.