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Well it can tell them if that person is heavily stressed by certain subject matter. More often than not that may be a helpful clue, but it still could just be their general nervousness in being in an interrogation room with cops hooked up to a crazy machine and being suspected of murder.
Again, polygraphs don't just measure stress. They measure changes in stress from a baseline. That baseline is established when you're in the room with cops. To establish your reactions when you lie they ask you questions they know you'll lie about and see what changes. So when they ask you questions they're not sure you're lying about they can see if you react the same as you did when they knew you were lying.
Most experts put the accuracy of a lie detector test at between 75% and 97%. Not good enough to use the results of one individual lie detector test to convict someone, but when you have two different people telling a contradictory story you know one isn't telling the truth. If one person passes and the other fails twice... That's a lot different.