A visualization of how much sleep I'm lacking in order to satisfy some basic sleep requirement would help towards alerting me to go bed earlier.
Also if I'm doing something to improve my quality of sleep (meditation, no digital devices an hour before sleep) it is rewarding to see that I'm sleeping longer and deeper.
Your reasoning can also be used to dismiss exercise tracking. You just exercised and you know you feel like you put in sufficient work. What's the point of knowing if you "competed a circle"?
For whatever it's worth, there are some studies that suggest that even if you work out every day, sitting for a consecutive 8 hours a day will still shorten your lifespan. So a watch might remind you that you've been sitting to long and to take a 5 minute stroll.
I don't know how well this works in practice, but I know that's the primary purpose of similar functionality in Sleep Number mattresses ("You didn't seem to sleep well; you should bump your number up to 63 and raise the back about 2 degrees more"), and I figure it would be the same with a smartwatch counterpart (though probably with less precision; it can detect the tossing and turning, but probably can't detect pressure points the way a smartmattress theoretically can).