What are the major reasons for the development and expansion of police unions, and what is their impact today?

Police unions, most prominently the Fraternal Order of Police (FOP), came into existence for largely the same reason many other labor unions came into existence: To protect their memberships from unfair labor practices. Police historically have been underpaid, overworked, and routinely subjected to dysfunctional and dangerous environments. While the salary situation has improved over time, the day-to-day activities of law enforcement, at least as far as patrol officers are concerned, remain inherently stressful. By its nature, policing involves regularly interaction with people at their worst, whether because a civilian was stopped for a traffic violation, police were summoned to deal with a domestic disturbance, or because police were dispatched to the scene of an ongoing armed robbery. All of these, plus myriad other routine responses, place police in precarious situations with life-and-death decision-making expected of all officers. In addition, many police departments experience periods of serious disconnect between rates of crime and available manpower due to budget constraints. When patrol officers are stretched thin, crime can rise and the physical and mental burden on officers increases dramatically. Organizing into labor unions was a reaction to these individuals' perceived need to protect themselves against forces, such as state and city legislatures, over which they would otherwise have no control.
Police unions continue to exist because the challenges facing police officers rarely change for the better. Especially today, when racial animosity between some police departments and many of the citizens whom they are sworn to protect and serve threatens public safety, the need for law enforcement officials to be represented by an organization that serves solely to protect and advance their interests is more important than ever. As noted, police officers regularly confront difficult situations that test their training and resolve. Knowing instinctively when to discharge one's firearm is a responsibility few citizens ever have to face. The wrong split-second decision can mean a criminal trial for the officer involved and the almost certain prospect of a civil suit by the relatives of the now-dead or incapacitated suspect. Unions such as the FOP provide representation dedicated to police interests that will help officers deal financially and emotionally when they do make that split-second decision and the result, justified or not, leads to public vituperation. Unions also help to ensure that police are adequately compensated in the same manner as other labor unions and are protected, to the degree practicable given the nature of the work, from undue risks to their physical well-being.