Large commercial airliners require two pilots, in order to share the considerable workload that exists in certain phases of flight, and to allow for the possibility of one pilot becoming incapacitated (they can be flown by a single pilot in an emergency).
Small private aircraft have two steering wheels (yokes) to allow two people to share flying duties, and in many cases to allow for pilot instruction, with an instructor on one side and a student pilot on the other.
Having two pilot positions is somewhat of a tradition in aviation, for the safety of redundancy. Some aircraft have only one set of controls, though, such as many military fighter jets and some very small and experimental aircraft.