It gives you a limited amount of control of a second app while you have another one one fullscreen. For example: You can use Quicktime to record yourself speak

It gives you a limited amount of control of a second app while you have another one one fullscreen. For example: You can use Quicktime to record yourself speaking to a set of PowerPoint slides, and then use the touchbar to end the recording without needing to exit the slideshow (leaving an ugly view of your desktop that you'd have to edit out with postprocessing afterwards).