Why is BOM so important in PCBA proofing and pasting?

Why is BOM so important in PCBA processing? In daily work, many customers who do PCBA processing will ask, do you have a BOM here? Why is BOM so important in PCBA processing?
BOM means bill of materials. A BOM of a product, indicating how many spare parts are needed to assemble the product. All PCBA manufacturers need to purchase the materials needed for this batch of production according to the BOM provided by customers, and evaluate the production cycle and process. It can be said that BOM is the top priority in all PCBA processing.
Since BOM is so important, what is its format?
At the beginning of general BOM, product model, version model, formulation date, document number, total number of pages, etc. should be indicated.
During the whole life cycle of every electronic product, from R&D to production, to improvement, the design will be continuously optimized or the functions will be expanded. When the new version of the product comes out, you must communicate with the PCBA manufacturer. The new version not only means the deletion of materials, but also the deeper problem is that the front-end design, including the schematic diagram, has been changed. At this time, all materials and quotations must be re-made, and the corresponding old versions must be marked as expired documents and sealed, and immediately stopped using. In order to ensure the orderly management of versions and models, the BOM list of products must be issued by the R&D department, at least signed and confirmed by the R&D engineer and R&D department supervisor at each level to ensure the correctness of the BOM list.
The relationship between BOM and products can only be that multiple BOM tables correspond to one product, but not that one BOM table corresponds to multiple products, even if the two products are only different in the content of one label.