The mushroom will not fruit on bare compost. ICAR-DMR calls casing a 3 to 4 cm layer of soil laid on spawn-run compost and a prerequisite for fruiting.
It works in four ways. It holds a reserve of water the crop can draw on. It lets carbon dioxide escape from the compost below. It supports the crop physically. And its bacteria give the signal that turns spreading mycelium into pinheads. What it does not do is feed the crop. Casing carries almost no nutrition, and a rich casing grows moulds rather than mushrooms.