This is the probably the most useful mode of this thing-
It's good for checking for obviously bad cables, but the huge advantage of this thing over using a test light or ohmmeter is the continuity test function which can catch very short intermittencies in the cable itself, the solder joint to the connector, a dirty contact, or simply a loose and worn-out connector that can otherwise be aggravating to chase down.
Plug in the cable, reset the indicator lights on the box with the push button, then wiggle the cable around every-which-way while watching for any indicator lights switching on. The lights latch-on until reset again indicating even a very quick break.