The goal of the use of a polygraph
in witness interrogation during criminal investigation (its original purpose, separate from all this bunk about “passing” or “failing” polygraph “tests”) isn’t to get the information the interrogation needs
from the polygraph, but rather to use that information to steer the interrogation toward whatever seems to most upset the witness, because that’s usually something they’d rather not be talking about (and ergo something that
may have involved them committing a crime, moving them from “witness” to “suspect.”)
Of course, polygraphs don’t guide toward lies specifically, just toward uncomfortable subjects—which is why you hear about e.g. TSA screeners demanding to know if you were cheating on your wife, when that has nothing to do with things the TSA could care about. Whether or not they’re using a polygraph, the interrogation process involves digging for upsetting subjects; and so, if you have something upsetting you unrelated to the case at hand, they have to “dig it up” to “weed it out” of the interrogation, i.e. to put the way you’re reacting to questions about the subject they do care about in context. A witness who is very nervous because they’re off to visit their mistress, has had their nervousness “explained away”, and so being nervous is no longer suspect. (But other responses, like anger, still are leading. Being an interrogator requires recognizing differences in subtly-triggered emotions, because they usually have different sources. This is why so much was made of the power of “recognizing micro-expressions” back when, before that got mostly debunked.)
And, of course, it’s pretty easy to train someone to control a polygraph signal, and some people are even naturally “immune” to being read by one. Mindfulness meditation works pretty well to defuse most of the physiological responses to stressful stimuli, for example, and interrogators have no idea whether you’re doing it. It works even better—if you’re a professional spy and have some time on your hands—to just sit around hooked up to a polygraph, watching its output yourself, as someone tries to upset you. (The polygraph is, in this case, acting as a https://en.m.wikipedia.org/wiki/Biofeedback mechanism.)
Still, polygraphs were never intended to aid in the interrogation of professional spies; they were intended to be used like a drug-sniffing dog, to act as a quick heuristic to filter a large pool of “ordinary passers-by” (e.g. in a murder committed in a crowd) down to a few suspects who seem genuinely uncomfortable about something. Since most crimes aren’t committed by sociopaths, or Zen monks, or spies, but rather by people who think they have a momentary opportunity and usually regret their crime somewhat afterward, looking for discomfort tends to be fairly helpful in pre-filtering a large suspect pool; and using a machine that can detect “silent” discomfort is indeed an enhancement to that pre-filtering. (But, again, it’s only a pre filter. It’s a weak heuristic that needs following-up with further interrogation and detective work, to know whether to put these people on an actual suspects list. It’s common, when using such a heuristic, to filter some people in on the first pass, but filter every one of them back out in a second pass, and then realize you have to start from scratch with a different approach.)
And, of course, there are going to be departments who know none of this, whose understanding of the use of a polygraph comes from cop movies, but who go out and buy one anyway, ditch the highly suggested training offered by the manufacturer (or don’t get access to it because they’re buying the device used), and then do stupid pass/fail polygraph “tests” where “passing” the “test” is exclusionary, as if you had a non-matching blood type to the suspect’s blood or something. (And there are also departments who just use a polygraph—real or fake—as a prop to perform “enhanced interrogation.” Interrogation doesn’t need “enhancement” IMHO—it doesn’t even need a stern voice—but at least interrogation with a polygraph-prop isn’t literally a crime and an abridgement of human rights in the way some other “enhancements” are.)