It's a psychological spiel. What a polygraph does is to note down reactions, both voluntary and involuntary. When you get asked questions, your body reacts. The idea is now that lying requires more "work" from you than telling the truth, since you have to fabricate it.
In reality, though, the way you react is dependent on so many facets that whether you lie or tell the truth plays a minor role, if any. It's like saying that you can tell what TV program someone is watching by looking at how much power he uses. While technically, in theory, possible, there are so many other appliances running in his house (or not) that their combination pretty much drowns out that information in way too much noise.
What is left of the polygraph is that people might THINK it works, and hence react differently. The goal is to give you the impression that it not only can, but WILL tell, without a doubt, that you're lying if you lie. So you get told that it can easily spot when and how you lie (it cannot), that it will be used in court against you (it cannot), that they used it multiple times to convict people (they have not) and so on.
The psychology around it is the actual "value" of it during an interrogation. Just like in medieval times showing the instruments of torture were usually enough to extract confessions, so does telling people about the polygraph. The main difference probably being that the instruments of torture can actually deliver what is promised, something the polygraph cannot.