It's a double edged sword.
1. The citizen will do the same. End result, you compare the two videos. If the police video goes missing, cases will be lost.
2. Bad or criminal behaviour by the police and no action, removes the trust in the police rapidly.
For example, Tomlinson. OK - policeman is a thug who killed someone. What's more worrying is his colleages who kept their mouths shut. Only when the video comes out, do we find out.
3. Criminal cases. Defence says, give us the video. Police won't. End result acquital if the video isn't available.