When lowering the pH in my pool, either just to get the pH lower or when doing the TA lowering procedure, I have a choice. I can calculate how much acid I need to have accurately using my spreadsheet or I can use the acid demand test in my Taylor K-2006 kit. I've done both, but over time, I tend to just use the acid demand test because it's there and it's easy. Eventually though, I just know how much I need to add (for regular lowering of pH, that is).

The other time the acid and base demand test is useful is when one is unsure of the pH reading. The acid and base demand tests let one move the pH color up and down to see where the pH might have been. I did that in the early days until I got used to the pH test though now I don't see the need for it anymore.