Polygraph interviews have at least 2 important functions.
1) The polygraph is a pretty decent prop. At least some people become more truthful, or at least worse liars, when they believe lie detection is happening.
2) Even if the truth of something cannot be ascertained at the time, if it ever comes out later, you have a very simple path to firing/imprisoning/otherwise destroying the person: they lied to a federal agent. This is a sigificantly easier bar to clear than actually proving they committed a crime. For this the polygraph is not actually important, but the interview itself is.
1) The polygraph is a pretty decent prop. At least some people become more truthful, or at least worse liars, when they believe lie detection is happening.
2) Even if the truth of something cannot be ascertained at the time, if it ever comes out later, you have a very simple path to firing/imprisoning/otherwise destroying the person: they lied to a federal agent. This is a sigificantly easier bar to clear than actually proving they committed a crime. For this the polygraph is not actually important, but the interview itself is.