Supplier Quality Assurance is there to help the organization manage/reduce risks when dealing with suppliers. Whether it's understanding the supplier's capabilities when considering order placement, seeking inputs to new product designs or establishing controls to avoid trainwrecks in production, all these are risks which if you don't handle effectively, will cost huge amounts of money and potentially customer concerns. An added benefit is that of supplier development, but this is dependent on the organization's objectives for their suppliers and available resources.
It's the role of (modern) SQA to identify, manage and reduce those risks.