I was instructed years ago to fire my top-loading ceramic kiln

A shelf placed on the top of the kiln will prevent brick particles from falling onto glazed ware. This is useful if the lid is cracked. It would not be necessary with bisque firings. Some people also place a shelf at the top of the kiln to prevent the incoming air of a downdraft vent from affecting glazed ware. The shelf is unnecessary if you drill the air intake holes on the outer edges of the lid and keep glazed ware 2” away from the holes. It would be better to fire the kiln without the blank top shelf since the shelf uses up more electricity.