Sometimes it is just impossible to have everywhere monitored by a person.  Even the best team of guards can miss something. But when an electronic alarm is in place, a location can be monitored 24 hours a day 7 days a week.  Using a combination of door sensors, window sensors, motion detection, glass breaking detection and many other sensors a premise can be secured against all intrusion.  And then the same place can be protected against other threats such as fire by having a fire alarm system that will watch the premises.  Combine all of this and you have a very reliable way to protect your assets.