With a used car you rarely know how old the battery is. The previous owner may have fitted it three summers ago. Most batteries carry a date code on the label or stamped into the case, and the technician can read it for you. Combined with a cranking test, it tells you plainly whether the battery has a year left in it or a month.
This matters more in a basement than on a driveway. A battery that fails outside your front door is an inconvenience. One that fails on P3, behind a barrier, with no phone signal, on the morning of a flight, is a much longer story. Battery testing at your bay takes a few minutes.