After 1850, the census had the occupation of at least the head of household.
In 1900, they asked how many years each person had been married, month and year of birth for each person, how many children each married woman had given birth to and how many of those children were still living.
Very useful information for genealogists, although the data isn't made public for at least 70 years (you can see the 1930 census now, and possibly find your parents or grandparents on it.)
That was because originally not everyone could vote, but districts were apportioned by population, with slaves counting as a portion of a person. That was a political compromise to keep the southern states from having the highest representation in the House. The other options were to count every slave (population would be heavier in the south) or count none of them (population would be heavier in the north). To balance they agreed on fractional representation for some.