When film is washed after fixation, Thiosulfate will diffuse rapidly out of the emulsion. At some point a stubborn level of Thiosulfate will remain embedded in the gelatin and this residue washes out only very slowly. The most speedy and effective way to get this residue out is through ion exchange. This is the reason, why prints should be washed in clean water first before they are immersed in HCA, and why deionized water is no good for washing.
Different anions are differently capable of replacing the Thiosulfate and must be at the same time not detrimental to long term stability of the photographic image. A test was made, and Sulfite came out on top, whereas salt water was less effective. Both were still more effective than pure water, and I guess in some regions salt water would be much, much cheaper than Sodium Sulfite. Note, that measurements have also confirmed, that after a fixer at pH >= 4.8 no HCA is needed anyway.