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 (e.g. the wrong kind of chip in an electronic device).
The pre-production inspection can also focus on the processes followed as production starts. Sometimes this can also be critical, as Chinese factories very often cut corners and do not respect the buyer’s blueprints (e.g. patterns for cutting fabric are received from the buyer, and they are modified to make the process easier and faster).