There are several reasons why a mult-port hub is necessary or useful: your computer only has two or three USB ports; you need to locate peripherals away from the computer; or the computer is set back on your desk, or on the floor.

To maximise battery life, notebook computers usually have just two or three USB ports. Desktop computers have more but, if you have a USB mouse, keyboard, and printer, you may only have one left. That's when you need a hub.

The more you work with USB peripherals, such as disk drives, memory sticks, mice, flash cards, card readers, digital cameras, webcams, and GPS systems, the more you need to arrange your workspace. So you'll probably need a few extensions. Don't forget them at the shop. Also, if you travel a lot, it's good to have a spare.

The computer's USB bus can provide up to 500mA at each of its ports, so low-power devices that would normally require a separate mains adapter can be powered through the computer's USB cable. The hub allows you to connect four peripherals, one of which could be a second hub, provided they don't collectively exceed the current limit.

With an unpowered hub, you can use any mix of cables, extensions, hubs, and up to 127 devices, providing the total cable length, computer to peripheral, does not exceed 5.0m.

It takes power from the computer's USB socket, so doesn't need a battery or a mains adapter. It uses so little power that it doesn't even get warm when it's permanently connected.