First, consider all the types of technology available for people and businesses these days. There are sophisticated alarm systems, surveillance cameras, and control access doors. These things, as well as other means of security, make stealing something--or harming someone that is protected by these assets-- very difficult.
Consider the advantages of having live, breathing people protecting valuables and buildings. Security alarms can be disabled. Surveillance cameras can be destroyed or tampered with. Control access doors can be fooled into giving access to the wrong people. Although these things are hard to do, many criminals still manage to do them. They have their own technology that gives them the power to disable, trick, or destroy security products.
Security guards, on the other hand, cannot easily be tricked or manipulated. If they are well trained, they should realize when a person is doing something suspicious or acting abnormal. Security guards are usually highly visible, which deters criminals from trying to steal something or harm someone.
The security guards can still be knocked out or killed, but will provide much more information on who the criminal is, than an inanimate security object. When a camera or security system is tampered with, they will not be able to tell who it was that did so. Security guards, unless outright killed, will most likely be able to report at least some information on the criminal or criminals. Also, they have the capacity to resist. They carry weapons and are trained to handle threatening people. Machines sometimes carry software that makes them resistant to tampering, but are still less in control over what happens to them.
There are some disadvantages to hiring security guards. For one, you have to pay them a salary. Security systems are more expensive to begin with, but less expensive in the long run. Also, there is the possibility of the guards being corrupt. A few dishonest ones may be bribed to look the other way while a criminal escapes. These are few and far between, but there is still the chance of hiring a disloyal bodyguard. Additionally, guards may miss something that a surveillance camera will not. They can be distracted or tired, and overlook a suspicious activity.
So are security guards necessary? Overall, security guards bring more benefits than disadvantages. Though they are prone to human weakness and exhaustion, they are still a definite bonus to any security system. It is recommended to have both a good security system, and security guards. Having both will provide even greater assurance that the assets or person is well protected from harm or theft.