Why do some helicopters have two engines

Is it mostly just for safety reasons only in case one engine fails? Or is it usually built into a helicopter most of the time for added power needed more than one engine can supply?
What is the MOST OFTEN reason that most twin engine choppers have two? Safety or some other reason? Do BOTH engines turn the rotor? Or is only one being used and the other is for backup only? Do you switch back and forth between the two? Or is there a main one and an emergency one?
Please advise about why a lot of heli's have two engines and how they are used and when..and in what order, etc..
Thanks in advance,
Perry in FL

Helicopters have two engines for more power and redundancy. For flying at night or under IFR commercially it is a requirement to have two engines. Some companies will specify two engines for certain operating conditions, like offshore oil rigs, low level patrols, etc.

Both engines power the rotor system at the same time and share the power equally between the two, and one will continue to power the rotor system should the other fail.

Same reason some aircraft have 1,2,3 or 4 engines. Some helos have 1,2 or even 3 engines.
More power available to use. Can still fly with an engine out, to make an emergency landing,.
Those the have two engines use them equally during normal flight.
Some with three engines, such as Navy and Marine. -53s use the third engine in emergency or very heavy lift operations.

In a twin engine design, both engines power the chopper at the same time. Should one fail the other will still be able to make enough power to maintain flight and land safely. Though very slowly.