Smartphones can be turned on / off, but the basic rule is to turn the power on and off. This is because you cannot receive incoming calls unless the power is on, and you will not receive SNS notifications. It is left in the so-called standby state, but when not in operation, it goes into a "sleep" state, and power consumption is minimized. However, it's a good idea to turn off the power once a week, even for a short time.
There are several reasons for this, but the most meaningful one is "initialization of system software (OS) operation". In both Android OS and iOS, multiple programs work together to perform various processes, but there may be problems with operation for some reason. Since the OS reloads those programs when it restarts, turning off the power can eventually return the system to a normal state.
Another reason is to reduce the burden on the battery. Even when in sleep, the battery is under load because the system remains running while the power is on. Unless it's charging, the battery isn't overloaded while it's turned off, so you can rest. If you turn off the power before going to bed, you will not feel any inconvenience.
But don't forget to turn it on. In the case of iPhone, you do not need to be strongly aware of power on because it will be automatically turned on when you connect the Lightning cable (start charging), but most Android devices will automatically power even if you connect the cable. It will not turn on. Be careful not to turn off the power if you were wondering because no one would call you.