It’s not just waste space: lots of gear storage in the tub.
For transporting gear and camping, you can fit about 10 full-sized tramping (hiking) packs into the tub, as well as the burner and flue.
The rigid lid padlocks onto the tub so it’s secure storage if needed. You get two good quality 40mm Lockwood marine padlocks for the lid as part of the standard Kiwitub kit.
You also get two plastic plugs to seal off the hose outlets against water, road dust, etc.