The AURA Strap 2 enables body composition analysis on the Apple Watch Series 3 through Series 7. Available in two sizes, the AURA Strap 2 has a six-month battery life, is waterproof and should be more comfortable to wear than its predecessor.
https://www.notebookcheck.net/Apple-Watch-Add-body-fat-muscle-and-water-level-tracking-functionality-with-the-AURA-Strap-2.619514.0.html
In my experience, body composition analysis on scales is just a gimmick. I don't see why this would be any different. It all depends on how close you are to their model. If you're not, it can result in complete nonsense. Meaning, it can depend on who is it targeted at. Is it targeted at obese people who are trying to lose weight? Or is it targeted at obsessive athletes? It can't actually measure the composition of your entire body. I guess you can use it for trend monitoring, assuming it gives repeatable results, but you can't take the numbers at face value. However, I don't see what is wrong with simply measuring or just feeling the thickness of the layer of fat.