You can use margin: auto to center items horizontally, but you can also use it to separate items in a flexbox layout.
If you’re creating a menu, it’s not uncommon to need the main portion of the menu on one side of the page and one or two links on the far side. Add an auto margin to the left side of the item you want on the far side (if the main portion of your menu is on the right side, put the auto margin on the right side). The automatic margin will fill the available space and push the two sections apart.