Why AC power adapter when the hub can suppose to be functioned without it?
With bus-powered mode (without power adapter) the hub will supply each device 100mA of power which is usually enough for most mice, joypads, and other low-power devices. But for some higher power demending devices such as digital camera, printer, scanner, 100mA is simply not enough. Once an AC power adapter is connected it provides 500mA to each downstream port, not only allowing those USB devices to work but also letting it able to make connection to other hubs as well.
