Salt has anti-bacterial properties. That is why it is used to preserve food, like pickles and meat.
The practice of rinsing the sinuses with salt water is popular in India, where they use a device called a neti pot. Similar to Just D, I use my hand, cupping as much salt water in the palm of my hand as I can and gently "snuffling" it up. Some water runs down the back of my throat, some comes into my mouth and I spit it out, some I blow out of my nose.
This recipe can also be used for rinsing the mouth and throat:
Dissolve the salt and baking soda in the hot water, then let let it cool to a comfortably warm temperature. This recipe approximates the salinity of our tears. What you don't use can be stored in the fridge.