They are sworn to up hold the law and protect society from wrong doers. In the modern day, officers are expected to present themselves in professional manner and follow a code of ethics know as "Police Ethics"". These ethics are often ignored due to pressure from fellow officers, feelings of being above the law and stress from quota deadlines. When these Ethic's are ignored it demoralizes the status of police officers and creates a potential liability issue. This effect's the tax paying community, the community that police officers are sworn to protect. .
When rookies get their job they must familiarize themselves with the other officers. This is important because these veteran officers are going to be his or her back up and it could be the difference of life and death. When entering a new station there is a possibility you will be told of a police code of silence, this means if you see something going on that is outside the ethic code you are to keep it quite. This will keep you in good terms with your fellow officers but could cost you your job in the long run. The smart officer will abide by the rules of law and expect his fellow officers to do the same. .
Another reason Police Ethic are sometimes ignored, is that the status of being a police officer goes to the officers head and they start to believe that they are above the law. This is one of the many reasons police officers are not appreciated by society. This only helps to feed the "Conflict Model"" which assumes that the systems subcomponents function primarily serve their own interest. This is damning to the demeanor of good police officers and takes away from seeking justice for criminals. So in the end you have internal affairs, which is, police policing, police which could be avoided if the original intentions of police work was kept in mind.
The last reason "Police Ethics"" will sometimes be pushed aside is due to the extreme pressure officers are under.