Add to Menus provides a quick link in your Wordpress Admin Bar to quickly add a menu item link for the post & page that you are viewing.
So you need to create a new post and a link in your menu to that post?
This normally takes a lot of steps; creating the post, going back to the menu manager, creating a menu there, selecting the correct post, etc, etc.
With Add to Menus this is done easily and quickly: Simply create the post/page and save it first. Now, a new menu will be there on the admin bar, you can click on ‘Add to Menus’. Select the menu you want to link to and voila! You are done.