When you would use an unpowered USB hub. Unpowered hubs draw power from the USB port they're plugged into. As computer USB ports produce small amounts of current, they are best used to power devices that need only low voltages, such as: keyboards and mice

USB hubs provide their own power supply for devices. Typically, when you connect a device to your computer via USB, the device draws upon your computer's power to function. Powered USB hubs come with their own power source and provide power for your connected devices so your computer doesn't have to