 is that originally NASA was not going to fly any missions out of Earth orbit without a lunar module. They wanted to have the lunar module as a redundancy, as a backup, as a lifeboat. And, in fact, that's exactly what happened on Apollo 13. When the service module on the Apollo capsule blew up and it was out of commission, they didn't have any of the oxygen or supplies from the service module, they opened up the lunar module, and that kept the guys alive until they could get back to Earth.