A clear bit of plastic that does what it promises.
It protects from dust and spills, good to keep on during gigs in messy places when you are not using the mixer, and seems pretty strong (I have not tested how strong for natural reasons)
In combination with the Allen & Heath Carry Bag QU 16 you seriously increase the protection for the levers and knobs during transport.
All motor-fader-mixers should come with such an addition as dust combined with moisture will reduce the life span of the board a lot.
Cheap insurance. A good buy.