Why does FedEx ask for form 5106 for imports to the US?

FedEx will sometimes request the completion of a form 5106 for U.S. Customs and Border Protection (CBP), when the importer or ultimate consignee appears to be unknown.

Form 5106 Importer ID Input Record - is requested when a formal entry is required for import clearance.
Once submitted it sets the importer/customs relationship. The importer is requested to provide their Employer Identification Number (EIN) or should that not be available their Social Security Number (SSN). In cases where an importer does not have an EIN or SSN U.S. CBP is able to issue a Customs Number.  

A formal entry is required where goods are valued at 2500 USD or more, or subject to Partner Government Agency controls or licensing (any value).