In automotive diagnostics, an AC coupling adapter is very useful because you can view certain signals more closely and thus simplify finding the problem. AC coupling removes the DC component of your signal so that it is oscillating around zero. This is advantageous because it allows you to zoom in more tightly on the waveform and measure small AC disturbances.
The main application is performing engine relative compression tests by observing the AC pulsations of the cranking current with a current clamp. You can also view the rectified voltage pulsations of the car alternator.