If you know the angle of each joint in a robot arm, finding where the hand ends up is straightforward. The useful question is the reverse: given where you want the hand to be, what angles must each joint take? That calculation is inverse kinematics.
It is harder because there may be several ways to reach the same point, or none within the arm's reach. Solving it well, quickly and smoothly, is what lets an arm move to a target naturally rather than lurching, and it underlies almost every deliberate robot motion.