why unity velocity is in vector code example

Example 1: unity how to set rigidbody velocity

Vector3 velocity = new Vector3(10f/*x*/, 10f/*y*/, 10f/*z*/);
GetComponent<Rigidbody>().velocity = velocity;

Example 2: rigidbody velocity c# unity

//for rigidbody2D
GetComponent<Rigidbody2D>().velocity =new Vector2(40,0);