Workload distribution
2) The other pilot (either Captain or FO) can hopefully recognize an error made by the other pilot.
3) Training. You don't come out of flight school ready to captain an airliner. The time in the right seat provides seasoning.
4) Reduction in fatigue. Flying every leg, especially as a single pilot would be incredibly tiring.
5) Medical problems. Something as simple as a kidney stone can be totally incapacitating. You wouldn't want 100-500 people to die because the one and only pilot got sick.