Stupid question, but how exactly does that help me on a 16:9 screen?
There are two parts to this answer.
The first is that we weren't really trying to add new features with this work. Instead, we were making the existing options more flexible, understandable, and universally available. For example, you've always been able to do some black bar cropping with DVD but not with other video types. And previously all modes were a single list, but this is a little confusing. You might have an anamorphic lens but still need to correct the aspect ratio of a certain video.
As for how 'Crop Black Bars' helps if you're watching at 16:9, it doesn't unless you're willing to crop away some of the video to remove the black bars. If you were watching in a little window or in Standard View > Playing Now with a wide display area, you might get black bars on all four sides and using this feature tells the program it doesn't have to do this.
As an aside about aspect ratios, I'm bitter that they finally convinced me to switch every display I own to 16:9 or 16:10 and now a lot of the best looking content is even wider so I still have black bars!