I first ran into this too many years ago in engineering statics. A head or column of liquid exerts  a  pressure per unit area only dependant on how high the column and the density of the liquid. No calculus involved to calculate the pressure at the bottom if the density can be considered incompressible such as fuel or water, but not air if the column is very tall. Now if we wish to calculate the total force on a dam or a container that has a vertical component, then we need calculus to be able compute the force. Do not have a neat analogy at the moment to explain it better.