However, as of the late 1990s Macs, it gradually was more and more useful. Before it was removed in the newest Apple keyboards (around 2002), it could completely turn on and off Macs connected to it.
Press command and the power key (Classic Mac OS only). You've just invoked the interrupt switch. Type G at the resulting dialogue box (debugger) -- if you can, that is, because sometimes the system won't wake up after an interruption via this switch (e.g. if it is seriously hosed).
Press command-shift and the power key (all Mac OS versions). The Mac will restart -- you've just invoked the reset switch. This tip is good for Mac OS X, too.