Older PCs had, despite their slowness and other shortcomings, in comparison with their modern descendants at least one important advantage: they almost universally were fitted with a switched mains output socket for the monitor. The main power switch on the PC controlled this socket, therefore: computer off = monitor off!
Modern PCs make use of a ‘soft’ power switch, which puts the power supply in standby mode only; as a consequence, the switched mains output on the back of the power supply is usually omitted. Progress, therefore, compels the user once again, to separately switch off the monitor by hand. Naturally, this is often forgotten.
Fortunately, there is an easy way to do something about this. It so happens that when the PC is switched on, a potential of +5 V is present at the game port. Therefore, it is enough to simply tie a relay to this signal, which then switches the monitor (and printer, etc.). This uncomplicated relay circuit restores an old convention: computer off = monitor off!