Adding folders to Favorites will allow you quick access to these folders. It is very convenient especially when you have multiple accounts with Outlook. Please follow the steps below for details (or see how to remove folders to favorites):
Step 1: In the Outlook message window, click the "Home" tab;
Step 2: Move mouse to the folder (e.g., Inbox) and right click to show the dialog box, click the "Add to Favorites";
Step 3: The "Inbox" folder is added to Favorites.