There are cases that you want more control to your menu
Suppose you want a specific menu with complex css, layout, images.
CFormatter is one of Yii's core components and used to format values displayed in widgets like CGridView or CDetailView. All classic formats like numbers, text, urls or datetime are supported but you may have to use a formatting rule that isn't built in. Let's say we want to display arrays of strings (like tags associated with a product model). When using CDetailView we would usually make use of PHPs implode function to create a simple string value of all values in the $product->tags array.
Its very common now-a-days to have Mobile app for web apps. For Mobile apps we need web APIs to fetch data or even post/update on web. We found a very simple method to make such interface.
Mobile App can call plain get or post request and receive data back in JSON format. JSON is relatively easy for mobile app to handle while plain standard GET/POST request for easy for Web server to handle. This mixed approach saves lot of time on server side. We not really need to handle actual REST requests and still be able to exchange data between web and mobiles or ajax java-scripts or even node.js.
If you want to edit data directly in the gridview in a 'dropdownlist' style, then you could use the 'type' => 'select' option of TbEditableColumn (YiiBooster) or EditableColumn (x-editable for Yii). This wiki shows how to get data from the db for the 'dropdownlist', by converting AR model data into an array that is passed in json format.