Im one of the collectors for the 2011 census.
"Central government, local government, the health sector, businesses and market researchers, academi reasearchers and the education sector, genealogists and the public at large rely heavily on the census results for a wide range of purposes:
-resource allocation : it is crucial that population estimates are accurate, consistent and comparable between areas.
-targeting investment: data much be consistent nationally to allow investment to be made in the areas its most needed.
-planning: basic population counts by key characterists, such as ethnic group, household type and size are important for planning.
-policy making and monitoring: there is a clear drive across government for policy initiatives to be evidence based therefore basic info on population size, age, sex, migration etc is important.
-research: the ability to produce statistics for small areas is vital for many research uses. Basic population estimates and counts by characteristic are also required."
Also, it's illegal if you don't complete it. You are obliged to fill it in by law.