Say you have a flex container with some flex items inside that don't fill the whole area.
Now I want to push that "Menu" item to the far right. That's where auto margins come in. If I put a
margin-left: auto; on it, it'll push as far away as it possibly can on that row.
Actually, you might consider
margin-inline-start: auto; instead and start using logical properties everywhere so that you're all set should you need to change direction.
Also, note that auto margins work in both directions as long as there is room to push. In this example, it's not alignment that is moving the menu down, it's an auto margin.