Census is not just about how many people live in the United States. It also apportions congressional seats, which means electoral votes.
Example: California has 53 congressional seats, mostly Democratic; Texas has 36 and Florida 27 mostly Republican. California's citizen population has been pulling up stakes and going to Republican districts like Texas, where taxes are lower and government less chaotic. Which means California could lose congressional seats. So California acts as a sanctuary state allowing illegal aliens to come in, even giving them free health care, driver's licenses, and help avoiding being removed by ICE.
But the plot thickens. It turns out there is an even more sinister plot behind their generosity and it is the focus of the entire Democratic Party. Counting everyone gives lawbreaking Democrats an advantage in gaining and retaining seats in the House of Representatives, which translates into electoral college votes that are winner-take-all. That means illegal aliens are in essence voting in our general elections because by default they have added to the number of electoral votes in sanctuary states.
Democrats have lost the presidency too many times in recent years due to the electoral college even though they won the popular vote. So if they can gain more seats in Democratic stronghold states they can take away votes from those states that don't support illegal immigration.