Why do PCBA proofing?

Usually, before a large-scale production, one factory or several factories will be selected for PCBA proofing. What is the purpose of this? The first is to detect whether there is a problem with the PCBA design process, whether there is any need to modify it, and the second is to screen out a strong factory to ensure the yield and test pass rate of PCBA production, and to evaluate the approximate delivery and quality. There can be an accurate estimate of the various situations of late bulk orders.