When we were studying motion in one and two dimensions, we assumed that we already knew exactly what the position function looked like. From the position function, we then were able to calculate the velocity and acceleration function.
In many real-life applications of physics, though, we typically do not know what the position function looks like, and would like to instead calculate the position so that we can predict the motion of an object. What we would like to know is how the forces on an object influence the motion of an object. How an object behaves under the influence of forces is defined by Newton's Laws of Motion, of which where are three.