Sensitive information doesn't have to be classified to be valuable to America's adversaries. Engineering drawings, technical specifications, and program data flowing through the defense supply chain can compromise national security if exposed, which is exactly why the government created a formal category for protecting it: Controlled Unclassified Information (CUI).
If you work with the Department of Defense in any capacity, understanding CUI determines your security obligations, your CMMC level, and ultimately your eligibility for DoD contracts.