In Australia, apart from being a headcount, it is one way of knowing who lives in the same house that are of age or nearly at age to vote. Like when our oldest child was about to turn 18. Then he got sent a letter and an application form to register so he can vote. These sort of census got computerized, so what is the excuse of other parts of the government "not knowing"? or no excuse for not sharing the info to other departments.
"Don't let the door hit ya where the dawg shudda bit ya!"
G'day and Glock
GATEWAY SWIFT WING ST. LOUIS