My understanding of copper was you could get theoretically all 4096 colours on screen at once (e.g. having 16 colours and changing each of them for every new vertical line - similar to a copper effect but doing it for all 16 colour registers... or even for 32?).
I never got to coding much extra with copperlists so I don't know the full capabilities.
But I also seem to remember you also could change a colour register's colour not only every line, but also after a few horizontal pixels.
One of the experienced coders can correct me on this if I am wrong.
I have no idea why some games didn't use copperlists more - did it slow things down a lot? (or even at all if it is only a small copperlist?)