transform.position related compile error fixed don't understand why ?

Trying to make a simple script to make enemy objects move left and right. Unfortunately getting error message.

script -

 using UnityEngine;
 using System.Collections;
 
 public class InvaderMoveBasic : MonoBehaviour
 {
 
     static bool goLeft = false;
     public int speed = 3;
 
     void Start()
     {
     
     }
     
 
     void Update()
     {
         if(goLeft == true)
         {
             transform.position.x -= speed * Time.deltaTime;
         }
         else
         {
             transform.position.x += speed * Time.deltaTime;
         }
     }
 
 
     void OnTriggerEnter (Collider other) 
     {    
         if (other.tag == "LeftWall")
         {
             goLeft = false;
         }
         else
         if (other.tag == "RightWall")
         {
             goLeft = false;
         }
     }
 }
 

the error message -

Assets/InvaderMoveBasic.cs(24,35): error CS1612: Cannot modify a value type return value of `UnityEngine.Transform.position'. Consider storing the value in a temporary variable

However when I change the script to this -

 void Update()
     {
         if(goLeft == true)
         {
             Vector3 TP = transform.position;
             TP.x -= speed * Time.deltaTime;
             transform.position = TP;
         }
         else
         {
             Vector3 TP = transform.position;
             TP.x += speed * Time.deltaTime;
             transform.position = TP;
         }
     }
 

Now things work fine, but I don't see why. The changes seem to do nothing more than add a couple of useless variables.

???