Video Switcher layer

In the setup of the Video Switcher Layer, there is an option to select some backgrounds. Can someone explain what this is for? It seems that if we can only dissolve or cut between cameras, there is no need for background. I could see, and would use a background if we were doing a wipe, as with the Placer with Transition layer (which sadly just has a black background during the transition) but I’m missing the functionality of loading backgrounds in the Video Switcher layer. Please advise.

the bg is for when you do a chroma key.

kmac1036 is correct: The background image inputs are used when you are shooting against green screen and your sources gets chromakeyed. For Example: you are able to create a virtual studio with multiple cameras. Each camera has to have a different background depending on it viewing angle on the scene.