Why do I need a additional cell line to use my Apple Watch?

If you own a GPS+ Cellular model Apple Watch AND you want to be able to use the cellular features of your watch, meaning you want to be able to leave your iPhone at home and still be able to receive and make calls, get text messages or email, then you need to set up a cellular plan with your cellular provider (assuming they actually support Apple Watch). If you don't need to have the cellular features active on your watch and you are fine with always having your iPhone with you for the watch to function, then you don't need to set up cellular on a cellular watch, which can be used as a GPS only model.

Most cellular plans for Apple Watch cost $10/month.

The Cellular + GPS Apple Watch has an eSIM and shares the iPhone's phone number, and the carriers that support the Apple Watch charge a monthly fee. In the U.S. the fee is $10 per month. Here is a list of carriers that support the Apple Watch:

https://www.apple.com/watch/cellular

The GPS only Apple Watch can do Everything the Cellular model can do AS LONG AS your iPhone is nearby, where nearby is generally somewhere in the same moderately sized home. The Cellular model allows you to leave your iPhone at home while you go for a walk, run, bike ride.

https://support.apple.com/en-us/HT205547

So you can share your iPhone's phone number for an additional fee on your iPhone contract, or you can get the less expensive GPS only model and let the Apple Watch use the iPhone's radios.