The most exciting thing to me about the 1940 US Census is that question about 1935. The clerk asked where each individual lived five years previously; and whether that was on a farm. They were collecting data about the Great Depression!
That period in American history saw a lot of displaced people. Wagon trains crossed the dust bowls with whole extended families seeking out better lives.
Within the last decade the demographic of the USA had altered beyond all recognition, hence this census being used to chart all of that movement.
For the family historian, this allows you a glimpse into the same journey. How did your own ancestors cope with all of that turmoil? Where did they come from? Where did they pause? Where are they now? The 1940 census will answer all of these questions.
Also on offer for your family tree is the usual information. You will learn where your family lived and if their homes were owned or rented. Moreover, you will know that house's value or how much rent was being paid. The emphasis again was on whether this was a farm. The crops were recovering and people were starving. The government needed to know where the supply was now!
For individuals, we will learn their full names, ages, sex, race, nationality at birth, marital status and whether they have attended a school. Educational attainment was a separate question. Had the national schooling levels slumped in the past decade, amidst all of the displacement, or not?
If people were working now, then what was their employment and was it dependent upon government subsidies? A lot of emergency labor had been made available in the 1930s, it was important to know how much workforce stability still relied upon it.
Finally salaries are discussed. For the 1940s US government, this was an indication of the next decade's economic prosperity. For the family historian, it shows how much your family had to live on in April 1940.