I would need to see the space to understand the question. Do you mean drywall on the studs or what?

Many custom closet systems are intended to be installed right over the drywall. Where the drawers are you won't see the walls, but you will see the wall behind open shelves and hanging clothing. I think it's only necessary to add a panel if you don't want to see the wall, but it seems money poorly spent to me. If you have wall damage, I would repair it and repaint before installing the new closet.

Thanks for your replies; i edited my question to add that the walls will be new and freshly painted. So I agree that back panels are not needed. Stay well!