Why AJAX Client Library Part 1?
If you are wondering why you may want to consider using the AJAX client library (forgetting about the AJAX extensions library server portion), there are multiple reasons to choose Microsoft's implementation.
Right now, on the internet, we are seeing a lot of different approaches to developing JavaScript. There are quite a bit of JavaScript libraries available on the internet outside of Microsoft's implementation (like JQuery or Yahoo YUI). Each of these implementations are designed to make JavaScript development easier and sometimes more managed. Microsoft's is the same; however, they have taken a neat approach to custom JavaScript development.
When looking at developing custom controls or extenders in ASP.NET AJAX, the client library helps create a client-side API for the control or extender that ASP.NET pages can make use of. Picture creating a control that can respond to events, can have property definitions to alter underlying data, or define methods that can be called. This all can happen on the client side without requiring a postback. In addition, its really easy to provide custom event handlers to events raised by client components, so the page can respond to an action of an AJAX component on the client side.
Furthermore, this can be taken a step further by having the client-side component re-render the interface whenever the data changes, especially when the data is streamed from a web service called through a proxy.
This is one of the underlying goals with ASP.NET AJAX, and I plan to talk about this in more detail on my blog. Stay tuned.