Jerky motion is a mechanical stress problem, not just an aesthetic one
A trajectory with abrupt changes in velocity or acceleration is not just visually unpleasant, it places real physical demands on a robot. A sudden spike in acceleration requires a sudden spike in torque to produce it, and motors have finite torque they can deliver, meaning a trajectory can be mathematically valid on paper while being physically impossible for the actuators to achieve in the time allotted. Even when the actuators can technically deliver the torque, doing so repeatedly stresses gears, belts, and joints, wearing out components faster than smoother motion would.
For robots carrying payloads, the stakes are higher still. A sudden acceleration can shake or destabilize whatever the robot is carrying, whether that's a fragile part on an assembly line or liquid in a container, and in the worst case it can cause a dropped payload or equipment failure. None of this is solved by a faster or stronger motor alone, it's solved by planning trajectories that are smooth in the first place.