Batteries are just one component in a complex electrical system. There can be much more going on than meets the eye, even when symptoms aren't immediately noticeable. Testing can detect the issue early on, allowing for a timely fix.
A good battery today doesn't guarantee its condition in a week, a month, or a year. Virtually every repair shop will have stories of a new battery sold or installed that had to be warranted immediately or shortly after. Even with the highest manufacturing standards in place, batteries are still susceptible to failure.
A battery test on the customer's equipment might show no issues when installed, but official running could have markedly different results. Because you simply can't know the usage, environment, and other factors, it makes sense to just run the test on every visit instead.
With the increasing dependency of backup systems on battery strings, and the escalating cost of replacing batteries, instrumentation and software systems that can measure, trend, and manage the life-cycle of cells is a cost-effective option.