I have AT&T/T-Mobile/Verizon. Why do I need an eSIM if the USVI is a U.S. territory?

 This is a safety measure against "accidental roaming." The USVI is mountainous and very close to the BVI (a foreign country). Your phone can sometimes connect to a BVI tower even when you are still on St. John, triggering expensive international roaming charges. A Cellesim eSIM locks your phone to the correct USVI networks (like AT&T) and guarantees you a fixed, prepaid price.