This extension could be used in so many ways. A few quick ways:
1. Grading students work. Have their work on one side and the gradebook open on the other side.
2. Writing a paper and need to do some research? Now you can have both open at the same time.
3. Last one - my favorite.... drafting for Fantasy Football but need your precious website up (you know that you tell no-one about and has helped you draft that sleeper in the 8th round). Well now you can have up that site plus ESPN or whatever website you use for fantasy sports.
Look below for a GIF and embedded YouTube clip.