I am no chemist, but I think photographers eventually figured out that fix didn't break down as fast when it was made acidic. The problem was that it didn't fix as quickly when adidified, which was resolved by increasing the concentration of thiosulfate. Highly concentrated sodium thiosulfate acidified with sodium bisulfite, potassium metabisulfite, or sodium sulfite and acetic acid produces a long-lasting fixer with high capacity and reasonable fixing times. I suspect that the acid plays a primary role in reducing stain from pyro (just as it does when you use an acid stop), but I don't think that is the main reason for acidifying fix.