They are there for both performance and redundancy. A twin engined plane must be able to take off even if one engine shuts down at the most critical time during takeoff. Of course it won't be a low noise takeoff for people near the airport but it will take off on one engine. Because of the reserve power required for twin engined planes, they have much more than enough power than multiengined planes of the same size. In normal operations when both engines are working, this allows them to climb faster to cruising altitude and decreases the noise around the airport.
ETOPS rules require twin engined planes that are flown more than 60 minutes from a possible diversion airport to have engines and other critical systems maintained by different crews. Mainetenance crews are not allowed to service both engines at the same time. This is done precisely to prevent the same mistake being made in maintaining them. It was improper procedures identically performed on all three engines during an oil change that caused the problem with the L-1011 in Miami. Many operators of three and four engined aircraft have started using ETOPS rules for maintenance even though they are not presently required, because it improves the reliability of they planes and cuts operating costs.