In days of old, no mechanic worth his salt would ever dream of charging a customer for diagnostic time. If he couldn't tell what was wrong by opening up the hood and looking and listening, then he might as well hang up his wrenches. Today's cars have so many smog controls and computer controls that diagnosing a drive ability problem is rarely simple. So mechanics have to charge for their diagnostic time, otherwise we'd go broke. But some people still don't think that a mechanic should charge for his diagnostic time, or that the diagnostic time should be applied toward the subsequent repairs.
Modern cars are very complicated.
Not only that, the technician doing the diagnosis has to have a tremendous amount of training, experience, and intelligence, and the shop has to have tens of thousand of dollars worth of tools and equipment to perform the tests necessary to figure out the problem. It's not just a fad. The mechanic hooks it up to various parts of the car and the machine instructs the mechanic to start the car, crank the car, run the engine at certain levels, etc. Finally it renders its diagnosis. Like I said before, diagnosing problems in modern cars requires a tremendous amount of training, experience, and deductive reasoning.