Both modules were needed to make the
astronauts survived not only the trip but also the reentry.
The Service module was what held all of the oxygen, batteries, and fuel
and other such nessecities for making the trip.
The Command Module held the instructments needed to pilot the space
craft after it had left earth orbit. It was also the
craft in which the crew would return to earth in because it held on its
bottom side a ceramic heat shield that would protect
the crew and craft from the frictional heat caused by reentry.