If you have ever had to clean out the inside of something after you've made a cut or done some welding you know what a pain it can be. First off just gaining access with a tool to do the job can be problematic second the splatter is molten when cutting or welding so it has a tendency to attach itself to the surfaces with a vengeance.
To prevent this just about anything placed between the weld splatter or the sparks from the plasma will prevent them from sticking to the surface since the sparks never make contact. But aluminum has one more advantage which is it is a great heat sink serving to reduce the molten sparks to cooler slag this helps to stave off secondary adhesion to another surface from ricocheting sparks
I used a pieces of aluminum angle bar to protect the 2 critical inside surfaces of the work piece
All of this would have had to be cleaned out