Often times a homeowner is in his/her vehicle driving and they cannot recall whether they shut the garage door. If the garage door is not shut while the owner is not home, the house may be easily broken into by a criminal.
Additionally, a homeowner may be in his/her home and ready to go to bed. However, the homeowner cannot remember if he/she shut the garage door. Again if the garage door is not shut, the homeowner's family may be subjected to intruders.