The crux of this BBC article on understanding the census is to be found at the tail end of the piece:

Sure, the Census is an evil conspiracy to pry, so that they, whoever they are, can know all about us. Until you see raw data. A good antidote to the evil-empire view is to come face to face with real-life counting. You soon realise that governments know half as much as they like to pretend, largely because gathering information is a bigger, messier, pig-sty of labour and guesswork, than often assumed.

Which is why they do it. Because they know a lot less than you probably think and always will. Every source of data is riddled with problems.

On the face of it, governments know a huge amount of things about the population. They’ll know how many claimants claim disability benefits and how many claim jobseekers allowance. They know how many people pay tax. They know how many people got a passport in the last year and how many people registered births, marriages and deaths.

However, they’ll never be able to know how many people fall into each category; maybe three claimants for jobseekers are the same person, and he also pays tax. Maybe someone has two spellings of their name and managed to get married twice. And I love that some people do this; I love that the system is never perfect; it isn’t here, it wasn’t in the USSR and I’m sure that there are even people off the grid in China.

Governments come and go, but the common thread is that people outwit them no matter what is tried. Which is nice.