Everything on a modern car runs through the computer. Climate control, entertainment systems, windows, lights, seats, various driver aids such as cruise control, lane-departure, blind spot warnings, hands-free driving, automatic/antilock braking, traction control, etc. as well as the engine and transmisison. Sure, something like a failing wheel bearing would still be diagnosed by the mechanic, but they need the scanners for most of the problems on a modern car.