Master the art of color correction, and learn how to edit images with challenging lighting issues. Make use of the wide variety of features that Photoshop has available for images with mixed light sources, complex highlight, and shadow situations, or underwater lighting. Gain the ability to bring images that you believe to be terrible back from the brink of the trashcan, and shoot with greater confidence that you can correct your images in post.
Good class as an add-on after taking Ben's Advanaced PS Class 2 - very worth the time. Especially lesson 5, makes the entire class very worthwhile. His technique works SO MUCH BETTER than Match Color under Image Adjustment menu.