Another reason to test pH is to verify a process. If an extreme pH is used to say, dye a fabric, or even generate synthetic fibers, then you'll want to check a pH to be certain a process is done. Obviously, if no one ever forgets a step, and the process always works, such testing would be unnecessary. But process checks exist in numerous products, from housing materials, to textiles to the most obvious, pharmaceuticals and foods.
I do you your methods are good and you understand them well. Taking the pH of a dry solid is pretty much impossible, and even a moistened solid is a little dubious. I don't know if that's what's confusing you.