If checking 100% of the order quantity, one piece at a time, would be very time consuming.

A much more cost-effective method is to draw samples at random and to inspect them carefully. Fortunately, all professional inspectors follow the same statistical rules (to ensure that the samples they draw are representative of the whole order quantity).

It is also possible to send an inspector when the first finished products are ready, rather than waiting until the whole order is completed. This way, you will be aware of potential issues early in the manufacturing cycle. It means the factory can take preventive actions for the rest of production, and you can adjust your shipment planning if needed. So this is why do random sampling.