Although you're not swallowing the Listerine the gargling of the Listerine does still kill some bacteria that is in your throat as well as your mouth.
So gargling with Listerine daily can help to prevent some illnesses by getting rid of that bacteria.
You can also gargle with Apple Cider Vinegar and even swallow some Apple Cider Vinegar so that it kills even more bacteria in your throat.
It has been shown in lab tests that Listerine does get rid of the gonorrhea bacteria while the salt water did not.