Because robots aren't enough and spacesips aren't enough and big futuristic architecture isn't enough. No, it needs a guy who shoots a robot with a gun. Plot:

For 500 years the distant Terran colony of Corwin had no communication with Earth. But now the invincible war-horde of the Klodni was on the march and the long-forgotten planet desperately needed Earth's help. Baird Ewing was appointed ambassador . . .

Yeah, yeah, yeah.