Why does my Mathf.SmoothDamp work for the Camera?

Could somebody explain to me why this code works? I don't understand why i need to ref a velocity vector.

     public GameObject player;
     public float smoothMultiplier;
     
         
     private Transform tr;
     private Vector3 camDistance;
     private Vector2 velocityRef;
     
         void Awake ()
         {
             tr = transform;
         }
 
         void Start ()
     {
         camDistance = transform.position;
 
     }
 
         void Update ()
     {
 
     }
 
         void LateUpdate () 
         {
         float newPositionx = Mathf.SmoothDamp(tr.position.x, player.transform.position.x, ref velocityRef.x, smoothMultiplier);
         float newPositiony = Mathf.SmoothDamp(tr.position.y, player.transform.position.y, ref velocityRef.y, smoothMultiplier);
 
         tr.position = new Vector3 (newPositionx,newPositiony,camDistance.z);
         }
 
 }

Vector2 velocityRef has a value of Vector2.zero right? Then why does my camera script work? Do I just have a crap understanding of C#? I thought when you reference something, that something needs to have a value assigned to it right?

I've read this article trying to explain this to me, but I understood only the first half of it and got super confused at the end. Ultimately I still have no idea why SmoothDamp wants a ref to an empty vector.