The rich get richer
Rich client functionality results in each page of the Web application acting essentially as a mini-application. Users can modify data and screen elements instantly by directly manipulating the UI on the their machines. Whether the user needs to add more rows to a table, sort the data, enter new data, edit multiple records, or copy and paste complete recordsets, they can do so quickly and efficiently on the client side, without the server delay.
Rather than serve multiple dumb pages, a rich client application can leverage its architecture to create smart pages. By optimally distributing the load across both client and server, users benefit from improved system response and an application performance boost due to better server resourcing.
Web applications can begin to break the chains of their thin client ancestors, which offer easier deployment of a networked application at the expense of user efficiency. With standards maturing and browsers' increasing compliance to those standards, UI designers and developers can now implement rich client functionality in clean and extensible ways, making life better for the users and developers alike.