It can always be turned off
It's like having an independant, unbiased witness in your car. If there's an allegation by a passenger it's their word against yours, if you're involved in an accident there's nothing to show who is at fault - unless you have CCTV. Could be something simpler such as a copper saying you went through a red light or didn't stop at a stop sign. Of course it can be used to prove guilt as well as innocence.
We had a case a few years back where a girl accused a driver of molesting her, he asked the police to tell her he had cctv fitted (he didn't but she didn't know that) as soon as it was mentioned she dropped the allegations.