Well, it is like an insurance, but very cheap indeed.
A true story: a colleague of mine, an expat working in a major, with a local/national copilot, had a situation on board, with some aircraft damage due to severe bad weather.
The extremely young copilot panicked, and the Captain ended up alone in the cockpit.
At the Company's board, the copilot lied about what happened in the cockpit, and the expat requested to hear the CVR.
But the Company "forgot to download it", and of course believed to the local/national, firing the expat Captain.
So, here comes one essential function of this cable: a personal CVR.
Plus, it is a astonishing truth-revealing to ourself: we are sure we said something in the cockpit to the other one, but when we hear to ourself, we discover how poor was our communication instead. Then this cable allows a deep a strong professional improvement, used also as a personal feedback tool.