In home appliances the part has to keep up with the pace of the assembly line. The same dimension across hundreds of thousands of pieces, the same surface and the same assembly behaviour are expected; a single deviating part stops the line.
In motor and pump bodies the alignment of bearing seats is decisive; in mechanism parts it is the friction behaviour of moving faces. Where the part is visible, the cast surface can be the final surface, so die surface maintenance is part of the production plan.
In serial production quality does not come from measuring part by part but from keeping the process steady: injection parameters, die temperature and cycle time are recorded.