The polygraph doesn't have to emit any useful data at all to be very useful in interrogations. Like a bomb doesn't have to have any explosive in it to clear a building. Interrogation is a head game and a complicated box with knobs and buttons and maybe even blinking lights makes a fine prop.

And there's enough ambiguity in it that it's easy for the operator to believe it helps. Like a dowser with their rods, a clergyman with a holy book or an astrologist with a horoscope. That gives them the power boost of sincerity.