The constituent equations governing a geometrical, physical, or sociological context are found by thinking geometrically or physically, etc., about the setup under question. The result of this thinking are equations that contain as variables time, coordinates of points, curvatures, speeds, accelerations, and more. Some of these quantities are derivatives of others, e.g., momentary speed is the derivative of the locality $x$ of a moving point with respect to time $t$.
It follows that a found equation $$\Psi(x,y, t, v,\ldots)=0\tag{1}$$ between the involved variables is often a differential equation, e.g., $y''-3y'-y=\cos t$. Thinking about a real system we want to know whether there are equilibrium points, or, under which circumstances will the actual solution $t\mapsto y(t)$ evolve to $\infty$, and how fast. In order to answer such questions we have to be able to solve the equation $(1)$. Unfortunately this is only seldom possible to do in finitary mathematical formulas. These are the cases that you have learned to handle "mechanically".