Buttons
Buttons are often provided, dedicated for a simgle (possibly broad) purpose. These include buttons to power off, adjust volume, reset the computer etc.. They provide a useful, cheap ever-present way of accepting input from the user, which is partly why they are not located on the screen.
Dials
Dials - simulated or otherwise - are very useful for accepting numeric input; and are therefore provided in case of clocks, timers, etc..