Proponents of these damned things, and yes, I am against their increasing intrusion on everyone's lives, say that they prevent crime. No, they do not! They record some crimes, and might make it easier for the police to catch the criminal(s), but not always. They can also be used for nefarious purposes.
A variation of the crime prevention theme is also used when CCTV is installed in workplaces. They are there for staff safety, so the staff are told, but there have always been suspicions that they would be/are used otherwise. Now the NASUWT has exploded the myth. It has evidence, in the form of e-mails, that CCTV is being used to monitor teachers in the classroom.
The teachers claim that they are the only ones being monitored in this way, but on that point they are wrong - we are all being watched, all of the time. Walk into any office, shop or bank and you can spot those little glass bubbles in the ceilings and walls. They cannot always be observing the baddies all of the time, most of the time they are watching us and the staff.
They are a form of control rather than an aid to crime prevention.