yii2 发送邮件 compose 创建 message instance

Question

下面代码中 composer 方法为什么同时传 html 和 text

return Yii::$app
    ->mailer
    ->compose(
        ['html' => 'emailVerify-html', 'text' => 'emailVerify-text'],
        ['user' => $user]
    )
    ->setFrom([Yii::$app->params['supportEmail'] => Yii::$app->name . ' robot'])
    ->setTo($this->email)
    ->setSubject('Account registration at ' . Yii::$app->name)
    ->send();