For Post-Production-Effects. You can use it for Depth of Field-Faking, and you can also produce depth-fog or depth-based desaturation (simply use the zDepth-greyscales as luma-matte for a solid colorlayer.)

Hahaha, that’s right..
It’s mostl used for post processing DOF effects using with an image editor like Photoshop. And through this, you can achieve DOF effect really fast, you can see the effect right away while adjusting. Isn’t that great? That’s the pure advantage of using z-depth in post processing over the one-pass render. ^^