Vector3.MoveTowards doesnt work

i am trying to understand about

 Vector3.MoveTowards();

so Vector3.MoveTowards(initial point, destination point, time) when i store the position of the object inside positionOne Vector3 datatype and call Vector3.MoveTowards(positionOne,nextposition,Time.deltaTime*speed); it doesnt work but when i do

 `Vector3.MoveTowards(plankTranform.localPosition,nextposition,Time.deltaTime*speed);`

it works. I dnt understand why. Is first parameter supposed to be initial point or active position of object. I have searched it on official unity manual but unable to find my answer... following is the full code

 public class BrickMoveVErtical : MonoBehaviour {
     public Vector3 positionOne;
     public Vector3 positiontwo;
     public Vector3 nextposition;
     public Transform plankTranform;
     public Transform positionBTransform;
     public float speed;
     // Use this for initialization
     void Start () {
         positionOne = plankTranform.localPosition;
         positiontwo = positionBTransform.localPosition;
         nextposition = positiontwo;
     }
     
     // Update is called once per frame
     void Update () {
         move();
         
         
     }
     private void move() {
    plankTranform.localPosition = Vector3.MoveTowards(positionOne,nextposition,Time.deltaTime*speed);
         if(Vector3.Distance(plankTranform.localPosition,nextposition)<0.1)
         { changeMovementPlank(); }
     }
     void changeMovementPlank() {
         nextposition = nextposition != positionOne ? positionOne : positiontwo;
     }                        
 
 }

can anyone explain why

 plankTranform.localPosition = Vector3.MoveTowards(positionOne,nextposition,Time.deltaTime*speed);

doesn't work.