When sodium bisulfate dissolves in water, if forms sufluric acid, in the proportion of 2.82 grams of the bisulfate is the equivalenct of 1 milliLiter of the acid in a 48% solution. This makes it less costly and hazardous to ship. It can be found in stores that cater to the spa and swimming pool crowd, as a pH buffer.
It is used in the formulae for the Fe EDTA bleaches, Kodak R-10 variations, and the Pyrochrome Bleach.
Here is what the Photo-Lab Index from Morgan & Morgan has to say about it: