look, there no getting away from the fact the automotive meaning of compound charging is fuck all to do with what happens in your anti-midrange-mobile, so dont try get smart

reason for compound charging with 2 or more turbos is 99% the ability to create enourmous boost pressures, sometimes over 250psi boost, with just isnt possible normally with a single turbo or multiples turbos in a non compound fashion, and with normalish spoolup and lower than normally expected backpressure too.

reason for doing it with a SC and a turbo is the above reason plus the immediate power of a SC and the top end power of a turbo, as well as the shockingly low backpressure.

reason for just twin charging (ie non compound) is the spoolup/responce/powerband bit, without the compound bit, and tho this means slightly lower charge temps, it means a lot more complex and slightly less performance advantage.