The cellular Apple Watch (if you activate the cellular radio) shares the iPhone’s number. Carriers that support the Apple Watch add a fee to your iPhone’s cellular bill. In the U.S. the fee is typically $10 per month.
If you get a GPS only, or do not activate the cellular radio, it will use your iPhone as long as it is nearby (which you most likely already know).
The cellular carrier has to track you Apple Watch radio, and route all calls to your Apple Watch, and route your Internet traffic, and pay Internet interconnect services to carry your Internet data to other networks, etc…. This is all in addition to doing the same thing for your iPhone’s cellular radio.
Paying $10 per month in addition to your iPhone bill is not much for what you are getting.