I'm not really imagining any "motivation" beyond the intellectual exercise of proving America spends lots of money on things it actually doesn't spend money on, by squinting until the whole world is blurry.

Economists would like everything to be dollars, because that means you can plug numbers into a spreadsheet and get answers that work. Centuries of this not working haven't dissuaded them and I don't expect my rant to change that.

People who actually think we should care about population welfare do not try to squint at the dollar figures like this paper, you'll see them looking at policies qualitatively rather than quantitatively, and this way you don't need to draw a dozen charts to come to the apparently startling revelation that the US is a wealthy country in which money is being stolen from the poor to be wasted by the rich and this is a bad idea.

Sometimes this sort of work will be excused on the rationale that it will guide political decision making. This is an error perhaps everywhere and certainly in the US. A Republican is not going to read this paper and say "Aha! My policies were mistaken, I can actually have better impact by reducing tax breaks on the wealthy and directing the increased income to the poorest". They're going to glance at it, conclude it does not support the policies they wish to enact and so it's irrelevant.