Using client-side scripts allows you to load a page once and change it during navigation, giving a feeling of "speed" to the user.
Javascript programming is not easy, and it is difficult to do, although using the framework you can redefine all cross-browser problems and crashes (I personally LOVE jquery ).
The proper use of AJAX calls allows you to download information on the server side without having to republish the entire page, which also creates a Web 2.0 feel for the user (you can use UpdatePanels in ASP.Net to reproduce this behavior, but not redo it ).
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