Moving smoothly, not teleporting
Knowing the start and end joint angles isn't enough to move a robot arm well. Trajectory planning fills in the path between those two points, deciding the sequence of intermediate positions, velocities, and accelerations the arm passes through as it moves. A naive approach might jump straight to the target angles as fast as possible, but that produces jerky, high-stress motion that wears out motors, shakes payloads loose, and can even damage the arm over time.
Good trajectory planning instead ramps velocity up smoothly, holds a steady speed through the middle of the motion, and ramps back down as it approaches the target, similar to how a car eases off the accelerator well before a stop sign rather than braking at the last possible instant. This kind of smooth profile, sometimes called a trapezoidal velocity profile, is a standard building block in industrial motion control.