One shape changes continuously into another, showing a transformation or evolution.
Product transformations, an object turning into a logo, outfit-change Shorts.
If you specify both the start and end frames, the model fills in the space between. This is the only exception to the rule of not specifying an end frame, so use it only when the change itself is the point.
Single continuous shot, locked camera: a watercolor painting of a red fox sitting on grass smoothly morphs over 3 seconds into a photorealistic live fox in the same pose, same position, and same grass, soft morning light, the brush strokes turning into fur. Use the painting as the start frame and the live fox as the end frame.
Curious Refuge — 8 Genius Ways to Use First Frame & Last Frame: VFX Morphing