Role of toString() in Java

/*
 * A program to erase the - in the middle of the following resident registration number and fill in the blank to print it out
 * Let's write it down. However, frequent string generation occurs by utilizing the StringBuilder class
 * You should try not to. 990925-1012999
 */

package ch11;

public class Ch11_2_Q1 {

    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("990925-1012999");
        sb.replace(6, 7, " ");
        System.out.println(sb.toString());
    }
}