It allows more gas exchange, in some cases, and also allows more evaporative cooling. I run tanks with and without covers. Either way can work, but a cover can cause problems in some setups. It's easier just to leave the top open.
Or a tall canopy. I have a tall canopy, but it had an open area on the back and a clown and swallowtail angel managed to jump out through there before I got some eggcrate and used that on the back of the canopy to cover the opening.
i think what they are referring to is the plastic grid used on ceiling florescent light fixtures sold at do it yourself home supply stores where they sell lighting fixtures. the grid has 1/2 inch openings.