Relational databases do not support inheritance so if we need to represent it, we have to somehow store meta info while keeping performance by minimizing JOINs. One way to solve this problem is using single table inheritance. All fields for the whole class tree are stored in a single table. Class name is stored in the type field of the same table.
Lets say we have such a CGridView widget showing a list of users for administrator. Users have status „active“ or „disabled“.
Grid widget puts class „odd“ or „even“ to rows and we want to preserve this. So we want to add a class „disabled“ to rows with disabled users.
Sometimes it would be nice if we could run yiic from an action, for example a migrate database button in our admin panel.
Or because you are on a host which doesn't have shell access.
Or maybe you are not allowed run popen or exec.
Most applications will have one or two sidebars and often you want to control the content that should appear in the sidebar based on the action. For example you will want to show something different on the homepage as opposed to some view page. While achieving this you generally also want to avoid putting the layout into each view, as that would mean if you wanted to change the layout (e.g. put the sidebar on the left instead of the right or do some funky layout which requires an additional div tag to be added) you'd have to go through every view, which does not achieve good code re-use. It is also bad practice to have lots of if statements in column2 layout just so that you can generate the sidebar correctly.