It does use the same number as your phone, other than Family Watch (for kids).
It’s useful if you don’t have your phone with you - accidentally or on purpose. I know some people like to run with just Apple Watch & AirPods, for example, and leave the phone at home.
Personally I do have Cellular active on my current watch, but I get a discount from my carrier so it only costs $5/mo. To me, the benefit probably wouldn’t be worth $10/mo but “just in case” I forget my phone, it’s occasionally handy. It also was helpful when I smashed my phone while on a trip; let me call people etc. until I got home and had the phone replaced.