A logic switch activates a function if another function is in a certain setting. You can "And" them, "Or" them, or "Exclusive Or" them.

Sometimes, you may want a mix to operate only if the landing gear is down, so you want to use a logical "And" for the mix. The mix will turn on if the gear switch is in the "Down" position AND the mixer switch is in the "On" position. People do the same thing with smoke...have it turn on only if the throttle is above a certain point AND the smoke switch is turned "On".

You can set an elevator compensation so that it works if the gear is down, OR the flaps are down, or both.