A Web page can contain only one ScriptManager control, either directly on the page or indirectly inside a nested or parent component. The ScriptManagerProxy control lets you add scripts and services to content pages and to user controls if the master page or host page already contains a ScriptManager control.
When you use the ScriptManagerProxy control, you can add to the script and service collections defined by the ScriptManager control. If you do not want to include specific scripts and services on every page that includes a particular ScriptManager control, you can remove them from the ScriptManager control. You can then add them to individual pages by using the ScriptManagerProxy control instead.