Question

why it is important to convert object into symbol in motion tweening

Answer 1

Motion tween is nothing but tweening a Symbol's movement from one position to another. 
To implement Motion Tween all that you have to do is, provide Flash with Symbol's initial position and the end position. Rest is taken care by Flash.
You can create motion tween only on symbols. So any object upon which you would want to implement motion tween, First convert the object to a Symbol.