Backpacks are quickly becoming as casual as purses. While traditionally used on trips when a lot of things need to be packed or protected from the elements, they're becoming more and more stylized by talented fashion designers.  

The can look more formal if they're on the stiffer side or have very high quality metallic buckles. Adjustable straps can dd a considerable sag which makes the entire outfit look more casual, and when the colors match your outfit, it makes the entire look seem like it has a lot more thought put into it, even if you're wearing really plain clothes.