(I thought these companies were only "real" in the movies.)
Private Military Corporation's (PMC) are made up of former military personnel (though from what I've read, untrained people work for these companies too), looking to use their military skills in the private sector. These companies are awarded government contracts, usually from the Department of Defense or Department of State, to provide services in dangerous areas that for a number of reasons, politicians don't want government workers. In the past, these companies have been used for drug eradication in South America, airborne and land based reconnaissance, and diplomatic protection.
So why does the government outsource this work when we have qualified government employees (troops, law enforcement, and State Department para-militaries) who can handle it?
The first benefit is the lack of oversight and scrutiny. As a private company, PMC's don't have to discuss their work with anyone and are immune from FOIAs that any U.S. citizen can file with a government agency. The government also doesn't have to discuss their "proprietary" contracts with the media, and contracts under $50 million dollars don't even have to be reported to congress.
Secondly, PMC employees don't have to honor treaties, declarations or government declared rules of engagement, as they are simply private citizens. Also, if they are killed overseas, the story doesn't get the media exposure that a serviceman would get if he were killed. As evidence of this, five employees of DynCorp (one of 35 PMCs in the U.S.) have been killed in Latin America in the last seven years and I have yet to hear anything about their deaths in the media.
Another benefit is that PMC employees, again because they are private citizens, can use illegal tactics, because they are not beholden to the UCMJ or other congressional statues, like military and State Department personnel are. This fact, while useful, makes it very difficult to control the men on the ground. (For example United States paid PMC employees in Bosnia operated a forced sex-ring. For more information on this go to the url below). And if an employee blows the whistle on abuses, he can be summarily fired, as only government agencies are covered in federal whistle blower laws.
As Jonathan Tepperman summarizes in his New Republic article:
... By hiring private military contractors... the U.S. government has found an effective way to conduct foreign policy by proxy and in secret. These proxies cannot be monitored, are effectively immune from all criminal sanctions, and are dangerously hard to control since they answer to corporate bosses, not military brass.
Finally, although this is a U.S. centric write-up, to be fair, many countries use PMCs to do their dirty work for them.
Tepperman, Jonathan D. "Out of Service." The New Republic, Nov. 25, 2002.