Hi Imikey,
Currently the default playback mode on Mac is in the background. This is because you don't usually need to look at your laptop anymore with you are connected to a mobile VR device and watching movies there. Running the video in the background utilizes much less CPU too.
However, when the Preview mode is turned on on your Mac SKYBOX. You will be able to share some content with your friends who are not wearing an VR device. The Screen Mirroring button allows you to turn on/off the Mac SKYBOX playback when it is in Preview mode without having to put down your VR headset.
The Mac SKYBOX used to NOT have the background playback option, so the Screen Mirroring button was added to lessen the burden of the CPU. Now this button seems much less necessary with the background playback option, but we are still keeping it just because of the aforementioned reasons.
Best,
Andy