Experienced coaches are big fans of this little part. This $5 battery extension cable can save big bucks down the road by keeping the power expander and cortex plugs in good shape. The cable is primarily meant to allow greater flexibility in where to put the batteries on the robot. One end of this 12“ cable gets plugged into the cortex (or power expander) and the other end gets screwed down to the robot somewhere convenient to where a battery is located; the battery is then plugged into the screwed-down end for the rest of time.
The real benefit of using an extension cable, and why many coaches make its use mandatory, is that it prevents wear-and-tear on the cortex plug or power expander plug. It's much cheaper to replace a $5 cable if it's damaged than a $50 power expander! While VEX can probably repair a worn-out plug on the cortex for $50+, its failure will undoubtedly occur when it's least convenient, like, say, in the middle of a tournament. Even in the lab, swapping out a cortex is a major event. (This author's team did not use a cable on their power expander in their rookie year, and the wires broke off the plug in the middle of CA State Championships. Luckily the team's pit-neighbors were able to lend one on the spot, but it could have been a total disaster!)
So please go and get your team an extension cable. Or two.