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.
Backoffice Grid Views often list information like Posts for a blog while showing at the same time linked information like the User who wrote that post.
For more efficiency, it is appropriate that the User is displayed as a link to the User detail page.