The thing we must consider as top priority is the safety and well being of the animals under our care. It is natural for birds to pick, dig, and explore with their beaks. This presents a problem because discarded food and droppings are no more desirable in a bird's home than in your own. One thing that helps keep a bird away from debris in the cage bottom is a grate. Many cages now come standard with this feature and they are available as accessories for most if not all others.
Grates can also be custom fitted to most any cage by cutting a piece of welded wire.
Such a device suspended above the eventual substrate reduces the chances of interaction between your bird and potential harm. The answer is simple. Bacteria breeds faster than we can clean. There is also significant danger from the ingestion of cage litter. Impaction of crops and digestive tracts are relatively common problems encountered by veterinarians as are both low grade and clinical bacterial infections.