Towing gear, bungee cords, ratchet straps, tools, gloves, jumper cables, etc.
Like a previous poster mentioned, easily removed if the extra room is needed.
I installed mine with stainless threaded nut-serts so I can also remove the brackets also though I don't anticipate having to.
GM gave me a $500 credit toward accessories for my truck due to my rear window fiasco and I bought both swing cases with it.