The Ext.Window is one of the most fundamental aspects of a rich Ext JS application. Many of the other big components within the framework, such as the grid and tree, can display information in an easily consumable and browsable manner, but the chances are that if you need to display or manipulate a record with a lot of information, do so without disrupting the user's workflow by navigating to a different page, you're going to want to use a window.
The great thing is that Window is flexible enough to slot straight into any application you may want to build with it. For ultimate power, simply fill it with custom HTML, or to create an integrated experience use Ext components. The fact is that the window subclasses panel provides you with the ability to construct complex pop ups by using the most suitable Ext.layout—be that of a form, or an accordion, or whatever—and have it look and feel the same as the rest of your application.
Although windows and dialogs will most likely have a place...