The census is specifically mandated in the US Constitution in order to insure that members of the US House of Representatives are allocated according to population. (Those who call the census “unconstitutional” obviously have not read the Constitution.) The courts have also ruled that state and local legislative bodies must be allocated somewhat according to population. So the census is an essential part of US democracy. Some extreme right wingers are urging people not to return the census forms; if enough refuse to do so the effect will be to allocate more representatives to states with fewer such right wingers. Talk about shooting yourself in the foot!
The government also takes the opportunity to collect additional information at the time of the census; it is not mandatory to respond to those additional questions but it helps our elected officials make intelligent decisions. The census bureau also conducts population surveys in between the decennial census, in part to keep the agency tested for the one that really counts, and also to provide additional information to governmental agencies. Typically only a small subset of the population will be surveyed.
One additional point is that old census records can be invaluable in tracing family ancestry. Many old census records are now available online through for-profit internet services who took the time to scan them in and index them. I’ve located several century ancestors in 19th century census records.