It gives prosecutors another bargaining chip in the plea bargain process, which is essential in cutting costs in an overcrowded court system. The number of criminal cases that are plea bargained (meaning the accused admits guilt in return for a lesser sentence or some other concession) can be as high as 80 or 90 percent of cases. With the time, cost, and personnel requirements of a criminal case, there really isn't much of a choice. The vast majority of people that are arraigned are in fact guilty of the crime they are accused. Even if you believe a defendant only deserves life in prison, without the threat of a death sentence, there may be no way to get him to plead guilty and accept the sentence. If a case goes to trial, in addition to the enormous cost, you run the chance that you may lose the case, meaning a violent criminal gets off scot free. The existence of the death penalty gives prosecutors much more flexibility and power to ensure just punishments.