I assume you are familiar with accessControl standard filter. It allows you to separate privileges to run some action from action code itself. There is however one problem with most common usage - you cannot use bizRules, at least based on some parameters passed to 'checkAccess' just because it is hard to pass them in accessRules() result.
After installing Yii basic application on our systems, most of the time we want to have multiple environment such as local, dev and live setup. Most of the times when we want to make changes to configuration file, we have to over-write configurations file for each environment.