There has been a posting on a conservative site about how illegal immigrants give the Democrats additional seats (14 ?), even if none of the ill
egals were to vote.
https://cis.org/Oped/Immigrants-already-tip-scales-US-elections-without-even-voting
Every 900,000 is a Congress person.
That is why illegals should not be counted for congressional reapportionment.
But census data is used for myriad other purposes as well, and a total nose count is needed.
The Bureau of the Census is one of the pillars of the federal data system. Congressional reapportionment is the reason it was established in the constitution, but it has grown far beyond that.