php - __toString - what does it do in this class?

Question

Most of the work I do is on a legacy app written procedurally. I'm beginning to convert pieces of it to OOP, and came across this example class used to add multiple times together:

At the bottom of the class, there is a public function called __toString(), which appears to return some formatted text.

class Duration 
{
    /*
        $d1 = Duration::fromString('2:22');
        $d1->add(Duration::fromString('3:33'));
        echo $d1; // should print 5:55
    */
    public static function fromString($string)
    {
        $parts = explode(':', $string);
        $object = new self();
        if (count($parts) === 2) {
            $object->minutes = $parts[0];
            $object->seconds = $parts[1];
        } elseif (count($parts) === 3) {
            $object->hours = $parts[0];
            $object->minutes = $parts[1];
            $object->seconds = $parts[2];
        } else {
            // handle error
        }
        return $object;
    }

    private $hours;
    private $minutes;
    private $seconds;

    public function getHours() {
        return $this->hours;
    }

    public function getMinutes() {
        return $this->minutes;
    }

    public function getSeconds() {
        return $this->seconds;
    }

    public function add(Duration $d) {
        $this->hours += $d->hours;
        $this->minutes += $d->minutes;
        $this->seconds += $d->seconds;
        while ($this->seconds >= 60) {
            $this->seconds -= 60;
            $this->minutes++;
        }
        while ($this->minutes >= 60) {
            $this->minutes -= 60;
            $this->hours++;
        }
    }

    public function __toString() {
        return implode(':', array($this->hours, $this->minutes, $this->seconds));
    }

}

The public static function fromString() contains a return, and is how the class is called from a script. How does this class use __toString? Why isn't the implode simply included in fromString()?