change has been put in place, and much of those tests are what we call regression tests. This really is no different than a mechanic taking your car out for a spin after repairs to test out to make sure the repairs worked and nothing else broke.
Unfortunately, in our IT-related industries, some of those changes and tests must happen at night so that normal, daily service largely remains undisturbed. The customer's convenience often comes at the discomfort of those implement and support it.