What is the function of a check valve on the inlet of a household macerator pump?
The check-valve prevents waste from flowing backwards after the macerator pump stops.

Macerator pumps usually can't stop backflow by themselves. The pump has to let some solids through so the pump mechanism isn't water-tight. If it was, the solids would wear the those water-tight edges down quickly and the pump would no longer be water-right.