For example, suppose your application displays a large table containing many widgets. Constructing and laying out all those widgets can be time consuming. At the same time, you need to fetch data from the server to display inside the table. This is a perfect reason to use asynchronous calls. Initiate an asynchronous call to request the data immediately before you begin constructing your table and its widgets. While the server is fetching the required data, the browser is executing your user interface code. When the client finally receives the data from the server, the table has been constructed and laid out, and the data is ready to be displayed.
To give you an idea of how effective this technique can be, suppose that building the table takes 1 second and fetching the data takes 1 second. If you make the server call synchronously, the whole process will require at least 2 seconds. But if you fetch the data asynchronously, the whole process still takes just 1 second, even though you are doing 2 seconds' worth of work.
The hardest thing to get used to about asynchronous calls is that the calls are non-blocking. However, Java inner classes go a long way toward making this manageable.