The short answer to the question of who is affected by police brutality is "everyone." Whether you are are the primary victim of police brutality, witness the brutality firsthand, or hear about it on the news or through the grapevine, you are a victim of police brutality. This serious crime is important to deal with harshly and swiftly.
Absuse of Power
The only thing that keeps people from overrunning police in our society is the respect that we have chosen to give police. When police officers abuse their authority, the people stop respecting that authority, and the result is an unalterable disruption in power structure. Disruptions in power structures always result in compromised safety for both police and populace.
As a result criminals act more boldly and with less fear of consequences. This leads to more shootouts and more car chases, which opens the door to innocent people dying or suffering wounds as the result of either bullets flying or cars crashing into them.
1985 Grossmont High School Walkout
As an example of an event where respect for the police resulted in a peaceful conclusion that could have gone badly, in 1985, during a heated contract negotiation between teachers and the school board, students at Grossmont High School in La Mesa, California, staged a protest in the form of a student walkout on behalf of the teachers. School administrators were powerless to get the students to return to class, and the school principal was assaulted by students who pelted him with oranges. The school administrators lost complete control of the situation, and 12 La Mesa police officers were called to the scene. Police informed students who had walked off the campus that they need not return to class, but that they were required to return to the campus.
The students complied, out of respect for the officers. While there was no respect for school administration, police officers (uninvolved in the dispute) who were both respected by the students and respectful of the students restored calm to the campus in short order and further mediated the situation in a way that resulted both in the students having their say and the administration restoring order when the students returned to class.
What Didn't Happen
Had the police been void of authority, 12 police officers could not have restrained 2,000 high school students with only their words. In the days before tasers and pepper spray, more officers would have been called, students would have assaulted the officers as they had the school principal, and the police would have likely responded with force, possibly even deadly force. Instead the students respected the authority of the officers. Authority is essential to the peaceful resolution of conflict by police.
Respecting the Law
Without respect for the authority that society gives to police officers, society itself risks collapse. That's why police must act respectfully, free from suspicion that they abuse power.
Thomas Jefferson said that all governments (of which police are a part) derive their authority from the consent of the governed. When police undermine our respect for them on the basis of brutality and unnecessary force, people are less willing to consent to the execution of law by the government, namely the police force. When that happens, everyone is affected, and everyone is made just a little less safe.