They provide cover for the food and water bowls, preventing contamination by droppings from above. To modify the cage for the safety of the persistent chewers, I make a wire box around the top and three sides of the nest box. It is secured by hinging it on the bottom using a spring to hold it in place. Wire is expensive, but it is poor economy to settle for 1 x 1 or 1 x 2 wire. It seems a challenge for all species of birds to discover just how small an opening through which they can poke their heads.
None seem to give any regard as to how to pull their heads back and serious injuries are the result.
The most labor saving way to purchase wire for a cage building is buying it in two different widths. I use the 18 inch width for the top and bottom and the 24 inch width for the sides, back, and lower fronts. YOU WILL NEED 1. A pair of wire cutters. Invest in a good pair, and if you plan on building many cages electrical wire cutters are a blessing.