Pretty much the only wristwatch I've worn in years has a built-in heart rate monitor. That's been great for answering how hard I'm pushing myself in a workout or long run. It also allows me to track progress as similar workouts result in lower HR than than they used to, or track decline after being sedentary for a few months. It's also good for keeping tabs on my sleep cycles.

That said, it's enough of a hassle that I don't feel like wearing it normally. I'd never want one just for telling time.