Leafing through a book from 1904, I came across the following interesting hint:

"The addition of Sodium bisulfite to the fixer (plain hypo) is highly recommended. Not only will it increase the life of the fix, but it will also remove the unsightly brownish stain that sometimes occurs when using certain developers (notably pyrogallol)".

Translation and rewording from German is entirely mine.

Now my question is: Is this the reason nearly all commercial fixers are acidic? To REMOVE the Pyro stain??? Or does someone know differently?

I fear this could be another of those weird traditions from the days of glass plates - like the standard sizes for printing papers...