In any web based application, a page that appear within the browser's window usually represents an interface of a single application. On the contrary, in a portal based web application, there can exists multiple interfaces that represents more than one applications within a single browser's window. These applications can be considered as running simultaneously within a single browser's window.
A browser window behave in such a way that, whenever user post or request data, it will reload the whole content of the window's page. In the case of portlet base application, where a single browser consists of multiple simultaneous applications, then all applications will be reloaded each time a request is made to only a single portlet application. Therefore, even if you are working on one of the interface, whenever you post or request data related to that application, the window browser will anyhow, refresh itself by reloading all other applications that appears in the window.
The LEBAH portal framework anyhow does not operate this way. By using AJAX, whenever a request is made inside a module's (portlet's) interface, only that module is being reloaded, while any other module's interface shall stay intact. In this way, you can works on many applications (portlets) that exists in a single browser's window, at the same time, submitting only the portlet you are currently working on, without affecting any other portlets that exists in the same window.