Lets give the word to zy0rg, the creator of post_depth plugin:
That variation is used to create the depth blur effects in 3d fractals.
Unlike Apo's native depth blur, with this plugin you can use any variation (I'd recommend sineblur) for blurring.
The drawback compared to native blur is that you need to place the blurred object "manually" (i.e. using ztranslate, rotate_x and rotate_y) so it might be a bit tricky to use. But I think that the result is 99.98% worth it.
You can download post_depth (and a few cool blurs) here:
A few examples of post_depth results:
To use post_depth, you will need par