I think people use them to: 1) figure out what's affecting their sleep quality, not necessarily just duration, and 2) trigger alarms within a window, rather tha

I think people use them to: 1) figure out what's affecting their sleep quality, not necessarily just duration, and 2) trigger alarms within a window, rather than at a specific time, when the device registers that they're between REM sleep cycles so waking up to the alarm is less shocking and unpleasant, and 3) have an alarm or pager-duty-type alert that doesn't bother anyone in the bed with them.