Confusion in deciding the units for game objects

Hi, I am new in unity and i have a confusion. May be it will be a silly question but i am confused about this. My question is why we multiply a vector with any parameter if we want to move a game object. Suppose i have a game object and i want to move it in direction of z axis. i will use these different codes

 transform.Translate(Vector3.forward * Time.deltaTime);
 transform.Translate(Vector3.forward *100);
 transform.Translate(new Vector3(0,0,100));

similarly when we add force to rigid bodies we use these codes

 rigidbody2D.AddForce(Vector2.up*300.0f);
 this.rigidbody2D.AddForce(Vector2.up*Time.deltaTime);
 rigidbody2D.AddForce(new Vector2(0,300));

There are many more scenarios where we multiply with a parameter.

why there is many different ways to do this task.. And most importantly why we always not multiply a vector with Time.deltaTime.

Simply i want to say how can i decide that i should multiply vector with any value or Time.deltaTime.