In order to use a cellular Apple Watch your carrier must support eSIM technology. Carriers in the United States supports it but it isn’t the same in other parts of the world. Local carrier Spark however just added support for eSIM, as well as Voice over LTE (VoLTE) in its One Number Wearable plan.
The plan costs NZD$12.99 (US$8.56) and gives Apple Watch owners unlimited data, though speeds can be throttled after 40GB. This allows Apple customers to leave their iPhone at home and just take their Watch with them.