I think those are valid questions to ask.
Article I, Section 2 of the Constitution states: "Representatives and direct Taxes shall be apportioned among the several States ... according to their respective Numbers ... . The actual Enumeration shall be made within three Years after the first meeting of the Congress of the United States, and within every subsequent Term of ten Years."
Section 2 of the 14th Amendment states: "Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State, excluding Indians not taxed."
Sure sounds like the idea was designed to count citizens who pay taxes and vote, to be counted for appropriation of funds and representatives deemed to serve.
As for what the motivation to bring back the question is (I'll ignore the word "antiquated" as that is your opinion of it, and an attempt to undermine the reasoning), it was never removed. It was on the "long form" of the census, which was moved to a "short form". When the short form was created to be used, that question was one of those left out. Trump is bringing back the question on to the short form.