They were doing calculations that could be done by keeping a running total.  Which meant that you could take a stack of cards and count, say, how many were men in Chicago between 25-50 years old.<p>See <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Tabulating_machine#Operation" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Tabulating_machine#Operation</a> for more.