I don't follow. What would it be testing? Why this instead of a hardware switch?
Often game developers put various hidden cheat modes into the code. For example pressing a secret combination or sequence of buttons will add 1000 lives, or give the player a large amount of in-game cash, or unlock all the levels, etc.
This makes it easier to test the game, preventing game testers from having to play through the entire game, just to test one part of it.