If you want to add a drop shadow to a clip, why do you need to use some other motion-related effect besides the Motion fixed effect?

What will be an ideal response?

The Motion fixed effect is the last effect applied to a clip. Motion takes whatever
effects you apply before it (including Drop Shadow) and spins the entire
assemblage as a single unit. To create a realistic drop shadow on a spinning
object, use Transform or Basic 3D, and then place a drop shadow below one of
those effects in the Effect Controls panel.