I'm sure you've been woken one night by the sound of an alarm ringing in your street and apparently nobody taking any notice. It's quite common in old alarm systems, especially those without battery backup or anti-tamper systems because they can be triggered for all kinds of reasons.
For this reason, many households put off installing them because they are of the opinion that they just don't work. What's the point if nobody takes any notice? The thing is, burglars do take notice.
Take a look at your street and see how many people have installed alarms. The obvious point here is that those who have an alarm will have an alarm box and maybe even an outside light. The light will probably be triggered by motion. Both of these things are deterrents, the noise the alarm makes is not.
Of course, some people decide that a deterrent is enough and instead of having a complete alarm system installed, will simply pop a false box on the wall. There are a number of problems with this approach, but the most obvious is that thieves have access to the Internet, too, and they can find out which alarms are fake very easily.
Statistics show that burglaries are mostly opportunistic. That is, a burglar will see an open window or door, or a lack of basic security and decide to have a go. The chances of them having a go are increased when they see something of value, i.e., worth stealing. The chances are reduced when they can see that by trying to gain access, an alarm is likely to go off.
And if an alarm does go off, it could obviously be connected to the police as well as the homeowner, making the burglary not so attractive after all.
Of course, the benefit of a professional alarm with multiple zones and protection comes into play when the thief has planned his deed. Although rare, some thieves have a particular reason for entering a house. Often they will be 'stealing to order' and their goal will be to find and retrieve a certain item of value.
A good alarm system won't just act as a visible deterrent, but it will also provide protection for such items and give intelligence as to where the perpetrator is, and even stream video footage of the crime in progress. Should the burglar get away before the authorities turn up, you then have valuable evidence that can be used to prosecute.