* It automatically calculates what time you need to go to bed if you're running a sleep deficit. (And it calculates how much of a sleep deficit or surplus you have.)
* Most useful of all, it gives me at least 1-2 days of early warning that I might be running a fever or flu - as I see my heart rate, which is usually quite consistent from night to night, leap 5 or 6 BPM over consecutive days.
This is especially important to know during the current outbreak. I can then do whatever I need to do early to shorten the length of whatever illness I might have (take vitamins/extra rest/fluids/etc.)
It also records a history, so you have a record of roughly when the fever starts and ends based on your heart rate, which you can then share with your doctor if necessary.