The Watch and Fitness + together has made a huge difference to my health. A couple of years ago I used my S3 Apple Watch and a sleep tracking app to identify that my snoring was having an adverse impact on my health and led me to getting a special anti-snoring mouthguard that I could then see a visible improvement in my sleep tracking app (and felt much better as well). I did some occasional swimming and walking, but never went to the gym. When Apple launched Apple fitness for the first time in my life I started doing regular daily exercise, bought a yoga mat and some dumbbells and since December have been doing at least 30 mins of exercise a day, every day. I feel much fitter, I have developed muscle tone in my legs, arms, and my core strength, sense of balance has improved. I have seen improvements in my Blood Oxygen levels (I upgraded my watch last year) and my Blood Pressure has improved from 158/99 to 133/83.
Both my daughters have Apple Watches as well and we all share our ring data, so it keeps me motivated to close my rings every day, and we all support each other through positive messages as we complete workouts.
I guess if you are already doing regular keep fit activities then the Watch may not make much for difference, but it has helped motivate and transform mine.