Zend Optimizer is an instrument, which is required to run files protected with Zend Guard - a popular app which is used to encrypt PHP 4 and PHP 5 files with the idea to protect them from tampering with the code or reverse engineering. Zend Guard is used by numerous organizations which create paid script applications, so in case you obtain such an app for your website, you will most likely need Zend Optimizer to be present on the server where you'll host it. If you're a programmer, you may also use Zend Guard to shield your code and ensure that your site visitors or clients will not be able to change it in any way. Websites which use Zend Optimizer generally perform much better due to the fact that their PHP code is precompiled, therefore it's already optimized and will be executed a lot quicker.