The rationale for continued use in clearance screenings is that it's not scientific, but it's better than nothing.
https://en.wikipedia.org/wiki/Polygraph#United_States
Interestingly (last paragraph of the link above) the US has indicted people for offering to teach other how to beat the test!?
Polygraphs as a power play - well, I'm sure it happens but that probably isn't the reason it is in the room. Maybe the polygraph is a ruse to make people think FBI questioners are stupider than they really are? I can see value in having a distraction from the actual questions.
Those are questions on the SF-86. Ditto for a lot of customs & immigration paperwork. "Idiot detector" questions.
But if you don't have them on there then they can claim "but you never asked".