The ScriptManager is the centerpiece of ASP.NET AJAX functionality. It controls the switching between release and debug versions of scripts. You saw how to create embedded script resources and register them with the ScriptManager to get automatic switching between versions.
The ScriptManager also provides access to the Authentication and Profile features of ASP.NET. This allows you more control over when and how you secure your application and leverage personalization data.
To participate in partial rendering with the UpdatePanel, controls need to use the script registration APIs on the ScriptManager. The ScriptManager can then control when and how the script is available in the browser to allow partial updates to happen and avoid reloading script unnecessarily.