I guess I am confused why you would need more than one test.
Locations that have variable water supplies or water quality will benefit from having the ability to test their current water quality just prior to brewing. The most important testing tends to be calcium content and alkalinity. With this information, its more likely that you can get closer to your intended mashing pH and sparging water target.
I like having an occasional report from a good lab like Ward, but if I had water that changed from time to time, I would rely strongly on test kits like mentioned here.