I do wonder if it's a Ph thing, and the salt is upsetting the bacteria the same way salt is used to manage yeast in bread and fermenting things? Maybe it's pulling moisture from the area and making the tissues less homey for the germs?

The Medical Internet TM had some ideas about how it worked, but since it confirmed that it definitely does help (and gargling with vodka does NOT, christ on a bike, folks, what are you doing to yourselves?), I'm fine with there being *some* mystery.