A pre-production inspection tells the buyer which kind of raw materials (or components) will be used. Factories are often suspected of lowering their costs by purchasing substandard materials, and this can be disastrous for the buyer.
A during production inspection (often called "DUPRO" in the industry) allows the buyer to have an idea of average product quality, early in the production cycle. It is the most useful and the most under-rated tool at the disposal of importers, who often only rely on final inspections.
The final random inspection is by far the most common type of QC check. It takes place once 100% of shipment quantity is finished and at least 80% is packed, so it can be a real random inspection (this is not exactly the case if quality is checked earlier) and suppliers cannot play games.
The container loading inspection, like the pre-production inspection, it is seldom used. But it can be a worthwhile option in some specific cases.