Any operator established in the EU customs territory needs one, and so does a non-EU operator lodging customs declarations or entry summary declarations there. Each holder has only one valid EORI at a time, issued by a single customs authority and used for customs clearance, statistical and security purposes across the EU. On a forwarder's desk it is the identity check behind the filing: an entry summary declaration into ICS2 stalls without a valid EORI for the filing party, so registration comes before the first EU-bound shipment needs its declaration. A VAT number is a separate tax registration: holding one does not mean holding an EORI, and holding an EORI does not mean holding a VAT number.
Glossary
EORI
The Economic Operators Registration and Identification number, which uniquely identifies economic operators to EU customs and is mandatory for the clearance of all types of customs operations in the EU customs territory, such as import, export and transit.