Why are fields Mappings Mandatory for Search and Match in Salesforce?

For Resume Field Mapping, you have mapped a value for the Index Custom Field for all Custom Objects i.e., Achievement Index, Qualification Index, Experience Index, Certification Index, Publication Index, Recommendation Index, and Skill Index are mapped. For more details, refer to Resume Field Mapping. These index fields are a unique identifier that must be mapped so that the resume data gets updated when the user update/upload the same resume (with same Email ID).