Salt is a natural cleanser...it has the properties to kill any harmful bacteria it comes into contact with. I am not a meat eater but I have heard of people soaking or rubbing pork, chicken and red meat with salt before cooking (to kill bacteria)...so for that reason and the reasons listed above, a solution of salt and water will help to heal any sores in your mouth, throat and wherever else needed. I personally use it on a regular basis as a mouth and nose rinse.
It is because it cleans all of the bad bacteria away without killing your body's own defense cells. So, you are keeping the infected area clean while your immune system works. That's why you should use salt water and not Listerine or other antiseptics like hydrogen peroxide or alcohol. Sure you are killing the infectiion but you are killing your immune cells also. The same goes for any infected cuts/sores on your body. Warm salt water is always best.