There are two types of answers to this question. One has to do with benefits that police officers have gotten from the unions that are similar to the benefits that any group of workers gets from their union. The second has to do with benefits that are somewhat unique to police officers.
On the one hand, police unions have brought their members many of the same kinds of benefits that all unions bring. That is, they have done things like raising the pay that police get and improving the conditions in which they work. They have improved the benefits that police officers receive before and after retirement. In short, they have made policing a job that can put a person squarely in the middle class.
On the other hand, police unions perform a more unique service. That is, they work hard to protect police from what they see as excessive pressure from outside groups. The police occupy a controversial position in our society and are often the target of great amounts of criticism. Police unions protect individual officers who are being accused of things like abusing their power. They also tend to fight hard against civilian monitoring of police departments. In these ways, they help officers by trying to keep them from being subjected to a great deal of scrutiny for their actions.
Different unions in different areas provide different benefits, but some common and more general benefits can be found in the majority of police unions. Unions generally protect police officers' rights to overtime pay when officers have to work more than 40 hours in one week. Many police unions also fight to obtain or protect pensions for police officers so that they continue to have an income after they retire.
These unions also protect police officers when they are accused of misbehavior or crime. For example, if an officer is accused of misusing his/her power or accused of a crime related to his/her work, that officer can be represented by an attorney that the union will provide.