They are used for intimidation primarily. If you are being interrogated by a police officer and gullible enough to believe in polygraphs then you are more likely to tell the truth.
However, I bet the effect wears off after the first interrogation because you lose that last ounce of doubt "what if it works".
However, I bet the effect wears off after the first interrogation because you lose that last ounce of doubt "what if it works".