"Why do we need special forces?" you might ask. Why distinguish some individuals as "special" when they too are just soldiers. You might take the stance of those who believe that special forces bleed off from conventional units' valuable resources needed to fight their own conflicts. Per capita expenditures per soldier in conventional units surely benefit from the economy of means inherent in larger training programs using fewer instructors and more students. You might take a stance against many special warfare units' confrontational style of leadership. Admitantly they are abrasive in their demands and requisitions. You might even say they shouldn't be maintained since they are only used in very uncommon specific situations. True that the call for special forces is not one that comes as often as, say, the National Guard to fight floods across America. But there is one thing you can not argue against.
Unconventional units work.
Look throughout history at different examples. The rangers of the French and Indian War (or Seven Years' War), The Office of Strategic Services in World War II, Operation Kingpin in the Vietnam War (which was conducted correctly, but had no POWs to extract still at the camp), The Sayeret Matkal Counterterroist Unit at Entebbe, the Special Air Service in the Falklands. This list could easily go on for several more lines, but the point is clear. Across historic periods, across national boundries, across different eras of warfare, unconventional thought and methods in battle have resulted in victories normal units could not accomplish.
The same is true in the 24th century. In this period conventional battles are usually fought in the depths of space and on worlds far away from soldiers' homes. But some situations have not changed. Counter-terrorism; anti-terrorism; strategic and tactical intelligence gathering; reconnaissance in force. Even in the day of the United Federation of Planets, these missions have not faded in importance. If anything, they are more important. And they are still ver present and require soldiers with extra training who modes of thought is not necessarilly in line with their bretheren in conventionall Starfleet. Given the high tactical and strategic value of starbases, realtime human intelligence, information concerning an enemy's internal state, and those who entrust their lives to their citizenship, special forces are important, needed, and though not commonly known, very utilized.
The Department of Starfleet Special Operations is an organization established to perform missions where the cost of life is sure to be enormous and the necessity for secrecy of operation is essential in circumstances needing specilized training not commonly practiced by normal members of Starfleet's ship-board crews. All members serve strictly on a volunteer basis. They can go as they please and can join at any time in their career, provided they pass certain physical and mental criteria.