I'm not a law-enforcement officer of any kind. I imagine CCTV helps identify the perpetrators after the fact, and in some cases the person monitoring them can spot situations developing and nip them in the bud. They often feature in crime-related TV programmes. For example, after the bombing in London last summer, the police were able to track the terrorists movements before the incident. A google search will turn up more info - for example, here and here.

I wasn't advocating CCTV, by the way, nor trying to get into a discussion of the politics. I was merely explaining the British slang word "hoodies", which someone asked about.