Shooting Banks and/or Custom Setting Banks ???

With all my previous Nikon cameras, I allways only used both Banks A, but now I'd like to use the 4 different banks ! I downloaded the " Nikon D800 Setup Guide" (at http://www.outthereimages.com/images/D800_Setup_Guide.pdf)and checked the different settings...

Now, can somebody explain to me -slowly and extensively- what each of the Shooting Banks and the Custom Setting Banks are used for ? Why TWO banks system ? Do I have to use both banks system or not ? ...can I use, for example, Shooting Bank #B together with Custom Setting Bank #A or #C or #D ?

I read Nikon D800 Manual as well as Thom Hogan's D800 guide, but I haven't found a CLEAR explanation on this subject !

J-P.