I know for a fact that it’s mandatory for people who reside in the US (citizens, visa holders or those with a green card) but does that also mean foreign visitors (like those from Mexico, Canada or EU) have to complete the census too when they are on US soil? Have you visited the US during a time a nationwide census was taking place as a foreigner (alien)? If so, do you have to answer the questions present on the census even though you do not live there?
To hazard a guess (don’t know for sure, not even for my own country), it’s for future historians to be able to build up a picture of who was where and when.
What might seem obvious and common knowledge now might not be in 100 years.
Much shorter term, the point of the census is to know where people are and what they need, so government spending can be done effectively. No point building hospitals/centrelink/etc in areas that don’t need them.
Australian census records are (opt-in) deanonymised after 100 years, so there is some historical value.