What is the advantage of having a seperate line for the watch apple 6

The Cellular + GPS Apple Watch 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 only way to get a separate line for the Apple Watch is to use Apple Watch Family Sharing, and then that is for a dependent child, which limits many of the Apple Watch features.

You could also use an iPhone with a Dual SIM/eSIM and get a send line on the iPhone, then pair that line to the Apple Watch, but that is rather expensive, when you can just share your iPhone's line.

If this is not what you were talking about, please clarify your question.

Via the iPhone "Watch" app, you can configure the Phone call notifications with respect to the Apple Watch. One option is "Notifications Off".

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.

Unless you want LTE data while away from your iPhone.