This is down to the power requirements of the peripheral concerned. Each USB channel has a power rating and capability which is normally 100mA per channel. Generally this is enough power for devices such as a keyboards, mice and PDA‘s.

Some devices, such as external disks and scanner often need more than the 100mA that is on offer. So to get round this manufacturers have been creative and chosen to join two USB channels in parallel. This will effectively provide up to 200mA for your device, and that’s why there are two plugs that go into your machine. If you only plug in one you may find limited functionality of your device since it would have insufficient power available.

This technique is not really recommended by the chipset manufacturers and not best practice, but in reality most of the time you can get away with it. But, should your peripheral require more than this, such as a printer or monitor then you would really need an external power source

However according to the specifications the USB 1.1 and USB 2.0 protocols limit bus supplied power to 2.5 Watts, in other words 0.5A @ +5V per port. The USB standard defines support for higher current devices, up to 500 mA, provided the device is configured and classed as high power but this is often not the case.