The Census plays an important role in the distribution of federal funds, social services coverage, and political representation. It’s important that it accurately reflects the American population — since there’s a chance that asking about citizenship will discourage certain populations’ participation in the Census, it shouldn’t be asked.
The current citizenship count gathered by the American Community Survey (ACS) is inaccurate. Including a citizenship question in the decennial Census will improve the count’s accuracy. It’s also not a major imposition, as many non-citizens already accurately respond to the citizenship question in the ACS.