If an alarm rings while the user is not around, the alarm will continue ringing until either the user notices, or the watch battery dies. This is undesirable, as it can disturb others in the household or leave you with a dead watch in the morning (which can happen especially quickly as the vibrator uses a lot of power).
It would be useful if alarms automatically cancelled after a given period of time, and perhaps notified the user of the missed alarm with a notification.