How much corn, potatoes, flax, sugar and honey did your great-great grandfather farmer produce in 1860? How successful was your great grandfather's small manufacturing business in 1880? How many men and women did he employ and what did he pay them? Where would a researcher find such revealing information about their ancestors? In the census — but not the one we usually refer to as "the census."
Sharon Tate Moody has written a great introduction to U.S. Special Censuses that I would suggest we all read. You can find her article in the Tampa Bay Online web site at http://goo.gl/2NbpE