 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. What's interesting, though, is that the decision was made to fly Apollo 8 to the moon in order to make sure we could beat the Russians alive.