Also, when you buy "12.5%" liquid chlorine at the pool store, it may be as high as 14.5% or it may have been sitting around. In which case, it could now be only 10%, or even only 6%.

Or YOU could have had LC sitting around and you want to know how strong it still was.

So you might be adding "12.5%" and measuring accordingly and only getting half the bang for the buck.

I dunno...it makes sense to me as an occasional test. I first came up with these formulae because I had to test my own LC in the big blue carboy and wanted to know just what I had.