Why use the complete overrides functionality?
The most common way to upgrade a component in Joomla is to uninstall it and then re-install it. This means that any code changes you might have made to the core files of the components could be lost if you don't have a back-up; it is also more difficult to keep track of the files that you've customized. With the code override functionality you can place your customized files in a folder outside of the JReviews component to keep all your customizations in one place and safe from upgrade losses. Of course you still need to check for file differences when there's an upgrade, because there might be bug fixes in your modified files, but now you can easily keep track of which files to check.