Most optical ignition switches still need a manual switch to turn their input power off because they have an idle current draw of around 10 milliamperes. If left connected to the battery without a switch to turn them off they can drain your battery to nothing in a week or so.

If you're thinking of an ignition battery eliminator that incorporates ignition power switching and voltage regulation, they do not require a separate switch because they get their power directly from the Rx, when you turn off the Rx they can't draw any current.