Suppose you need to make an animation of a 6 degree-of-freedom robot. However you only need to show the end-effector moving from point A to point B to point C, etc. You aren’t concerned with the movement of each joint at the moment.
Have you ever tried free-dragging something with six degrees of freedom? I guarantee you will have it flipping around and inverting due to the fact that there are too many positions and orientations possible. As you are looking at a 2-dimensional screen, the robot could be at any depth; and it’s also possible to satisfy the position with the joints inverted.
So how would you go about animating this robot? You could run some inverse kinematics to determine each joint position required for the desired position of the end-effector, then add motors to each joint, or you could figure out some distance/angle mates that you can control in the animation. But in the end, you just wanted a simple animation showing the end of the arm moving from here to there with all the joints and arms to follow without flipping.
Here is a video showing the steps of how to quickly create an animation with mates to the end-effector at various locations.