You need a card list for two main reasons:
MPC Autofill makes things very easy, but requires some time investment to set it up for the first time. Luckily this only has to happen once and every subsequent order can skip this page entirely.
Create a cards.xml, which contains the right art and cards for the order you want.
Sit back and let Autofill do all the heavily lifting.
The step by step process of using the MPC website and all of its subtleties.