[Magento] Why the catalog promotion rule disappear after some time?

In Magento, catalog rules apply to catalog and products to cut the product prices and promote sales. This is one special sharp tool for holidays promotion. Catalog price offers four ways to discount the price.

   public function dailyCatalogUpdate($observer)    {        Mage::getResourceSingleton('catalogrule/rule')->applyAllRulesForDateRange();        return $this;    }