David, I suspect purity is related to use.
In silver halide emulsions, the gold used must be pure of all other heavy metal contamination or you can get problems. For example, iron contamination at any time in the making of silver halide emulsions is very bad.
So, purity above some level may be important depending on use, and the type of impurity may be important as well. This is why analytical and reagent grades are supplied with a certificate of analysis on the bottle.