为什么在Java中String类是不可变的或者是final的？

public class StringImmutableDemo {
   public static void main(String[] args) {
      String st1 = "Tutorials";
      String st2 = "Point";
      System.out.println("The hascode of st1 = " + st1.hashCode());
      System.out.println("The hascode of st2 = " + st2.hashCode());
      st1 = st1 + st2;
      System.out.println("The Hashcode after st1 is changed : "+ st1.hashCode());
   }
}

The hascode of st1 = -594386763
The hascode of st2 = 77292912
The Hashcode after st1 is changed : 962735579