Why would an applicant who had been to the US before, but needed to?
A valid visa does not necessarily ensure entry to a country. The immigration official at airport can refuse entry if they are not satisfied with the information provided. Similarly having a valid visa does not mean a new visa will automatically be granted. If any material change occurs in the applicants situation between two visa applications, the visa decision may be affected. The fact that an interview has been arranged despite having a valid visa (albeit on lost passport), seems to indicate the embassy is not satisfied with one or more aspects of the application.