The idea behind a Post-Processing Volume would allow for creators to introduce various new elements via materials to affect the view-space.
Examples of this could be Vignette (Dark edges around the screen) without the use of attaching textures with scripted transparencies on a Heads Up Display.
Color Grading such as changing the entire color of a particular scene.
Post-Processing could also be driven via scripted objects as well. This could go a very, very long way in terms of editing various scenes and providing tons of new and interesting effects for people.
There is also a potential market involved for selling materials directly to people who need various Post-Processing Effects.