My first reason is if a police know that he or she is on camera they will become model police and use less force on someone who is being arrested.
Because that can mess their whole career up they can go to jail or get fired. New York and Los Angeles has opted to allow police officers to view the camera footage if an case is brought up against them in court. By doing that they're trusting the police officer to tell the truth about harming a criminal while they were being arrested. But critics say that giving officers a glance at the recordings can report dishonest reporting. Officers it is feared will tailor their reports to fit the captured images, revealing only what could not be plausibly denied and leaving out the other important details.
My second reason is, Police Supervisors can keep an eye on their officers to see how they handle dangerous situations compared to how they handle peaceful situations. Police Supervisors want to keep an eye on the officers so they can see how they improved since they joined the force and they can spot different ways to stop brutality and harassment towards citizens. Supervisors can see if the officer needs to be condemned or commended based on how they've been doing they do in the field. After Supervisors watch the body camera of the police officer they can give constructed or bad criticism based on the job they've been doing.
My counter claim is that what if someone that is being arrested knows they're on camera and starts attacking the officer that will make the officer become aggressive shoot the suspect. Which will cause protesters and news to slander the officer who was just trying to protect himself for a violent criminal that was trying to hurt or possibly kill him.
Body cameras are needed to keep everyone safe that's why I support body cameras.