Volatile with synchronized together

Is there any reason to use volatile and synchronized together in this code?

public class Helper {
  private volatile int n;
  private final Object lock = new Object();
  public Helper(int n) {
    this.n = n;
  }

  public void setN(int value) {
    synchronized (lock) {
      n = value;
    }
  }
}

Class Helper must be thread safe. I've got this example from the "Java Concurrency Guidelines" book, but it is still not clear: what is the reason for using volatile and synchronized together in this example?