Outlooks Favorite folders are a "must-use" feature if you have multiple mailboxes or folders.  It allows you to keep the folders you use most right at the top of your list.

I have seven different mailboxes set up in my Outlook. One is Microsoft 365, several are shared mailboxes, and others are IMAP accounts.  The left had bar on my Outlook can be quite long. Favorites keeps me from scrolling through the entire left bar.  I mark the folders I use frequently (including all the inboxes) as favorites.

If you don't see favorites at the top of your folder list, you may have to turn it on.  Here's the instructions from Microsoft that include New Outlook and Classic Outlook.

Whether you're using new Outlook or the classic version, adding a folder to your Favorites is as easy as a right click - then select Add to Favorites.  If it's already in your Favorites, right click and you'll see the option to Remove from Favorites.  It's very easy and a big time saver - especially if you have multiple emails accounts or shared mailboxes.