What is the purpose of BackgroundImage filter input in SVG 2?

[RazrFalcon]

https://www.w3.org/TR/filter-effects-1/#attr-valuedef-in-backgroundimage

In SVG 1, BackgroundImage was used in tandem with enable-background to render part of SVG into a filter input buffer.
In SVG 2, there are no enable-background anymore (no one really supported it anyway) and the spec simply says:

This keyword represents the back drop defined by the current isolation group behind the filter region at the time that the filter element was invoked. See isolation property.

But the filter attribute always forces group isolation, meaning it the same as using SourceGraphic, no?

And browsers do not support it either. Chrome does handle BackgroundImage sometimes (probably simply fallbacks to SourceGraphic), but not BackgroundAlpha.

What is the difference between SourceGraphic and BackgroundImage in terms of SVG 2?