My wife got that with hers. It's $10/mo with our carrier. She likes it, but I think she'd readily admit it's a luxury. My watch has the cellular radio, but I haven't set it up with my carrier. You can make and receive calls with any Apple Watch as long as it's in range of your iphone (handy if you're just in a different part of the house). If you're the kind of person who would carry their phone when out and about no matter what, there's no benefit to it. It just means you can leave the phone at home and still be connected. Obviously your watch isn't an ideal phone, but you can use airpods or any bluetooth headset with it.

I get the impression that the way this works is that the carrier sets up a different phone number for the watch, but does some forwarding magic so that it seems like you just have one phone number that you can use on two devices.