What are the benefits of gargling with salt water?

There is a natural tendency for a substance to move from a region of high concentration to a region of low concentration (see osmosis). This means that even water moves from its region of abundance to its region of shortage. Since salt water is more concentrated (less water) than the contents in your mouth (including any germs in there), when you gargle with it, it tends to draw water out of bacterial cells (which have more water in them). All cells are over 70% water and require water to function effectively and hence survive. If the salt water is concentrated enough, it will kill most germs in the mouth and some from the throat.