Often, purchasing and manufacturing vendors are the same. In cases they are not, M3 allows you to deliniate between them. Purchasing vendors are those to whom you’ve contracted for specific services and will be invoiced. Manufacturing vendors are those who the products you’re purchasing belong, though sold through a 3rd party purchasing vendor. An example would be purchasing Cisco Equipment Support (Manufacturing Vendor) through a Value Added Reseller or VAR (Purchasing Vendor).
Do I have to identify the manufacturing venoor and, if not, why should I? No, you do not have to classify product that originate from a manufacturer to that manufacturer. The reason to consider this, if you are purchasing (example above) Cisco Smart Net from multiple 3rd party VARs and would like to eventually consolidate your spend, tracking products by the correct vendor makes reporting and threshold alerts easier.