http://www.techradar.com/news/apple-watch-3-has-a-major-connectivity-flaw-but-a-fix-is-on-its-way

The big feature of the Apple Watch 3 that sets is apart from the others is its built-in eSIM that allows for LTE connectivity. 

Ideally, this will allow to leave your iPhone 8 at home and continue the adventure on the go. But as The Verge notes, the cellular service provided by the wearable isn’t all Apple said it would be. When out in the wild, the Apple Watch 3 preferred to seek out random Wi-Fi signals rather than the cellular signal that you have to pay an additional fee for.