Its intended to flush the digestive tract from one end to the other.
If the proportions of salt to water are in a proper ratio it will become the same specific gravity as your blood and when consumed will not theoretically be absorbed by the body and will flush right through from one end to the other for the most part.
In other words when drank it should go right through the system within minutes.
Its really only good while doing a Master Cleanse and probably the only time needed.
Its use is discouraged by many professionals for different reasons, one being the fact that it washes out all the digestive necessities including probiotics.