The answer to the second question is obvious: because you can't see both at once.

As for the use cases, here are my most common layouts (3 monitors - old 4:3, 1440p, vertical 1080p):

- preview/debugger | code | docs (programming)

- slides | video call | chat (online presentations)

- gallery | photo | same photo but 1:1 zoom (photo editing)

- effect controls | timeline,preview,more controls | clip bins (video editing)

- chat | game | game wiki