What was the purpose of Henry?

Henry was asked to join the staff by Stanton. He eventually became part of the inner circle as an advisor. As the narrator of the film, he was essential.

You make a good point.

I suspect Henry was hired because they needed a black face, as Henry himself suspected, and because his grandfather was a respected public figure. Also, he was just really smart. They needed young smart people like him and Daisy to build the campaign with.

He sat and kept quiet a lot of the time because he was conflicted about what they were doing. Manipulating Willie and his family, digging up dirt on a flawed but decent opponent, etc.

Henry is 'US'. Henry takes 'US' on the political journey, he is the character that represents 'US' as viewers.

Henry starts out as young and idealistic (don't we all?), and over time he is exposed more-and-more to the cold, hard realities of just how unethical and corrupt the US political system actually is.

All of Henry's beliefs and understandings of the world (especially the world of politics), are not only challenged, but destroyed, and Henry (the young idealistic kid who had a vision and a passion), is slowly reduced down to nothing more than an 'enabler' for the corruption of Stanton.

At this point it's probably worth mentioning that Stanton and Susan also took the same path to disillusionment as Henry, and, eventually became what they always railed against.

Henry's character is also there to show us how a person can get swept up in the whirlwind of politics, and how, even HIS perception of what is right and wrong becomes blurry when that's all he is exposed to, and how 'one-eyed' he becomes in support of Stanton as he is slowly seduced by the excitement and adrenaline it creates in him (these things can be very addictive - especially when the stakes are so high).

Like Libby, Henry was an idealist, but unlike Libby, had the ability to make the transition from 'idealist' to 'politician' (two very different spheres of life). Libby remained idealistic and Henry defected.

Henry decides, in the end, that he can't go the way of Libby, because it doesn't achieve anything, and he doesn't want to end up with a gun pointed at his own head, so he toes the 'proverbial' line, but has no loyalty to Stanton, and will whistle-blow on his extra-marital 'comings-and-goings' to bring him down if it suits his own agenda...it's almost as though he's keeping it in the wings until he needs the ammunition against Stanton to save himself politically.

Henry essentially becomes as corrupt, manipulative and self-serving as the President he now hates. That's politics baby!Especially in America.