In the aftermath of the fatal Apollo 1 launchpad fire in January 1967 — among the most egregious examples of sacrificing safety and reliability to schedule — NASA learned the hard lessons of the avoidable tragedy, pulled up its socks and built some of the most reliable machines ever devised by engineers.
Among them was the one component of the American lunar landing that lacked redundancy, a design imperative for manned spaceflight: the ascent engine that would lift moon walkers off the lunar surface to rendezvous with the orbiting mother ship, the Apollo command module.
Virtually every component of the Apollo Saturn V and spacecraft had redundant systems, often multiple redundancy. Space is deadly, the engineers and astronauts were always looking for ways to reduce the inevitable risks.
Backup systems were an obvious choice.