When you wake up exhausted (or not) you can see what/when contributed to that.

I used such for a couple years. Best part was specifying what time range I wanted to wake up, and it would watch for the best actual time to wake me. Much happier mornings then.

I gave my old watch to my girlfriend and having her use it for sleep tracking has been super helpful in tackling her sleep issues. Once she saw her numbers for deep sleep every night, it kind of kicked her into action to address the root cause. Sometimes just quantifying something is really helpful. It's also been somewhat useful in comparing the various approaches (prescriptions, marijuana, vitamins, exercise, etc etc), because sometimes it's hard to judge how good of sleep you got one night if you're constantly getting terrible sleep every other night.

There is a business school cliche: what gets measured gets managed

For me, it helps figure out if my sleep apnea is getting worse and I need to talk to somebody about my CPAP machine or not.