This idea can be useful in many ways. Define specific areas in your space such as a reading nook or eating area with just a portion of he wall painted. It's also just a great way of introducing some colour when you don't want to paint an entire room.
Place painted foam board in the back of the bookcase, giving the look of a painted bookshelf w/out the commitment. Love this idea for a small book shelf next to couch. Good way to use yellow without committing to it in a big space.