Why does this switch exist?

Wal-Mart has by far the best price on propane, so I'm there with the rest of the fearful masses buying stuff to survive the impending storm. Across from my lane one of the cashiers is closing down, and keeps having to tell people to move along. At once point I hear: "Lanes 9, 10 and 11 are all open... though John hasn't turned on his light yet." And my cashier hits this switch: And it occurs to me that I have always wondered "why?" The register is already a piece of powered equipment, has been for well-nigh a century. Generally, the operator has to enable the device; now by signing on with their own credentials, but in the past at least with a key. So, why a separate switch? Why not just have the light come on when the register is in "ready to accept transactions" mode? Sure, I can come up with cases where the register is being used for administrative tasks, but that's an exception, and an over-ride can be provided I guess. But the primary case by far is being missed, badly. Now, off to brave the ice weasels.