If you use a machine that is not Wireless - it needs a power supply unit or a battery.

When to use a power unit and when to use a battery?

The battery is lighter, and in most cases - more convenient. Only works with rotary machines. It must be taken into account that the used machines and battery connections coincide. By default, RCA connectors are used everywhere, but there are exceptions.

The power supply unit typically has a much longer service life. Magnetic/coil machines can also be connected to it. Power blocks do not have insurmountable compatibility problems with any machine.