Do you need one? No.
Why do I use an Edge 1030? It’s bigger, it has a solid mount. It can be accessories, silicone case, leash, it has more memory, It seems easier to maneuver the menus, I can charge it and keep it clean while still wearing my watch. I can also still be an Apple Watch victim too. There are other features that the Edge does that might not work on a watch. Most of which I would never use. (There were people that had iPhones on their bars, and they look like they can take flight!)
But the big screen makes everything look, well: ‘BIG’.