In many cases developers of one project use their own local environments for coding. There may be a common test platform and, of course, production. All these environments usually have different DB credentials and directory structures (first of all, path to YII framework directory). Another feature is debugging. It's convenient to turn it on for local environments, eventually - on the test server and never (generally speaking) - on the production.
You may have scenarios, where you want to store a specific array configuration to the database. A classic example could be saving Yii widget configuration to the database and then retrieving it at runtime, with dynamic parameters set. This wiki discusses one of the approaches to do this. Let's consider you have something like the NavBar configuration below to store in the db: