What is the Denoising Depth output for?

When enabling Denoising Data in the render passes the render outputs 3 additional channels.
2 of which, the Denoising Albedo and Denoising Normal, can be used as inputs of the Denoise Node in the Compositor however the 3rd additional output the Denoising Depth can’t be used by the Denoise Node.
And neither the doc entry about the Denoise Node nor the doc entry about the render passes even mention the existence of the Denoising Depth output.

So why is it here and what is it for?

It’s a leftover from the old denoiser, that some users found useful for other purposes so it was not removed.

Oh ok, but what does it show?

Here is a demonstration of our use-case for the denoising data render pass: Cycles feedback - #856 by Kyraneth