Commandbar actions eliminate the need to use the update notification\event to update\change the state and properties of an item that appears multiple places in your menus and toolbars. All that is needed now is a single line of code placed almost anywhere in your application to update all control that contain the same id. For example, you might have a menu item that also appears in a toolbar, then a user might have created the same button and placed it in a user created toolbar. With Actions you can simply modify the action for the item and it will update all occurrences of the item, no matter how many a user might have added. This also makes it easy to localize all the control with a single line of code.
In the diagram below, the "Menu Item," "Toolbar Item" and "User Customized Item (user created toolbar)" all share the
same "Item Action." When you modify the properties of the action it will update all occurences of the item.