yii2 ログは何をするのですか?

'log' => [
    'traceLevel' => YII_DEBUG ? 3 : 0,
    'targets' => [
        'error' => [
            'class' => yii\log\FileTarget::class,
            'levels' => ['error', 'warning'],
            'enableRotation' => false,
            'logVars' => [],
        ],
        'article' => [
            'class' => 'yii\log\FileTarget',
            'categories' => ['pay'],
            'levels' => ['error', 'warning','info'],
            'logVars' => ['*'],
            'logFile' => '@runtime/logs/'.date('Y-m-d').'_article.log',
            'prefix' => function ($message) {
                $user = Yii::$app->has('user', true) ? Yii::$app->get('user') : null;
                $userID = $user ? $user->getId(false) : '-';
                return "[$userID]";
            },
            'enabled' => true
        ],
        'order' => [
            'class' => 'yii\log\FileTarget',
            'categories' => ['order'],
            'levels' => ['error', 'warning'],
            'logVars' => ['*'],
            'logFile' => '@runtime/logs/order.log',
        ],
    ],
],
\Yii::info('start calculating average revenue', 'pay');