You can use the Power button on older Apple USB keyboards to turn on your computer when it is connected to a USB port on your computer.
You cannot use the Power button to turn on your computer when the keyboard is connected to a USB hub or to a USB port on your monitor. You can also turn on the computer using the Power button on the computer. If you have an Apple Display with an ADC connector, you can also use the power button on the front of the display.