Utility of background-composite and background-blend-mode?

What is the difference between background-blend-mode:<MODE> and
mix-blend-mode: background <MODE>?

What are the use-cases for 'background-composite'? It seems like a very
specialized feature whose effect can be achieved pretty easily by just
using more elements or (in some cases) pseudo-elements.

Rob
-- 
Wrfhf pnyyrq gurz gbtrgure naq fnvq, “Lbh xabj gung gur ehyref bs gur
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tvir uvf yvsr nf n enafbz sbe znal.” [Znggurj 20:25-28]