So on the ARCameraBackground component there are 2 options. Use Custom Material and Use Custom Renderer Asset I understand that Custom Renderer Asset is used with the LWRP. But can anyone explain and give an example of why you might use a Custom Material?
Custom material ARCore and ARKit use different shaders to render the background. If left null, the material is created for you by shaders included in those packages. This option lets you override the default behavior. Custom renderer asset This is to support LWRP. There are some branches on the samples repo on github that show how to set this up.