The conic sections represent relations between the squares of two variables or between one variable and the square of another. Thus, they are relatively simple relations to understand. For example, the very simple relation represented algebraically by
[MATH]y = \dfrac{k}{x} \text {, where } k \ne 0[/MATH]
is represented geometrically by a hyperbola.
Moreover, the relations represented by the conic sections arise frequently in nature. For example, the motion of an object under a gravitational force (absent friction or other complicating factors) is described either by a parabola or an ellipse.
As for the theory behind calculus, the conic sections are irrelevant.