Just because the numbers aren't available.
It should be pretty obvious why these things are useful. They keep both the police and the public honest in their interactions.
If they are mandatory for a force, then an officer will have difficult questions to answer if they are turned off, especially if during an arrest.
Conversely, if someone is arrested and claims the police acted improperly, the footage can show whether this is true or not. This is especially true in cases where mobile phone footage turns up on YouTube showing police restraining someone forcefully, but mysteriously doesn't show the run-up where the person being restrained is committing the act that led to their restraint.
Obviously they have limitations - they can only record what is in their field of view, and anything happening off frame is open to interpretation. Overall though, I think their use is a positive influence on justice.
The flip-side is that people have legitimate concerns about being filmed. I'm not sure why this should be different because it is the police doing it, rather than anyone else (it is perfectly legal to record in public places, with some exceptions around sensitive areas). I'd suggest that the footage from these things should be properly kept, and be available to FOI requests. Obviously, there are considerations here where it may be evidence in a court case, and releasing it publicly before that case is concluded would be prejudicial to the case, but I don't see any reason why forces shouldn't eventually release all the footage into the public domain. If nothing else, it would give people a good education as to what the majority of police work involves, because I bet you it's not what most people think.