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.
It usually takes place once some finished products have come out of the lines. If quality issues are found, what is already produced might be re-workable, and corrective actions can be taken for the rest of the job. It gives buyers the time to plan ahead, and even to avoid delays (repairs and re-inspections take much more time when problems are noticed after all production is finished).