What is the purpose of torture?
First of all, it is not primarily for the purpose of gaining information. Professional investigators know that information is more quickly and accurately obtained by other methods.
No, the purpose of torture is to disorient and ultimately to break down the victim to the point where they are no longer effective opponents. Those who break may be released into the general population, where they serve as an example to all who would oppose the regime. In effect, they become an instrument of state terror themselves.
The Nazis used torture to terrorize the population. So did Pinochet. So did Stalin and Mao. They knew that the threat of torture was more effective than any secret police force in keeping the population in line.
Now the president has the authority to declare American citizens "enemy combatants," and therefore eligible for "enhanced interrogation."