Having metrics like heart rate, calories burned, and time remaining would show me clearly where my body is in addition to how I physically feel on screen which could be a huge help for me. Add factors like sleep tracking and I have an even better idea of how much I can push myself before I workout.
Unrelated with this announcement: there’s a lot of value to an Apple One bundle now (Apple TV, Music, Arcade, Fitness, News) but I have too much brand loyalty to Spotify to switch to Apple Music and feel like I would hurt publishers by using Apple News instead of keeping my individual news subscriptions.
On the other hand, if they offered free 2-day shipping with Apple Pay for Apple One subscribers at a bunch of premium retailers and bump up the price, you’ve got the first competitor to Amazon Prime.