It's preservative properties? It kills bacteria if used in high enough concentration (ie pouring it on). I assume we're talking about your average Sodium Chloride salt here. It is also essential to life, as we require the sodium in our cells- we need the ions in our cells to provide a positive charge for depolarization. Bearing in mind that our very being is dependent on electrical signals (essentially we are organic machines). Mind you, I don't know if I'm 100% here as it's been about 5 years since I last studied physiology and biomechanisms...
I'd say the preservative property is the main reason- because it kills off terrestrial microbes (or at least slows their metabolism right down!) it can be seen as clean or cleansing. Possibly the same as iron and silver etc. The Arabians supposedly used to carry two silver coins in their water flasks as the clinking together of the coins was said to cause a small charge that was just enough to kill off potentially harmfull microbes. Whether this is true or "auld wives' tales" I'm not so sure, but there might be some truth in it- we all hear about hte healing properties of silver, which is down to its bacteria-killing tendency.
There might be something in Marian McNeill's The Silver Bough. I will have a look.