The one thing I would miss with an Open Box motherboard would be the I/O shield. Maybe it will come with one but like they say, there's no guarantee. You don't absolutely have to have one but it gives the PC an unfinished look. You can sometimes get them from the motherboard maker but it will take time and since you have to remove the motherboard to install it, it's inconvenient. My 2 cents. edit for spelling error.
It's the little panel on the back of the pc that keeps the connectors from being completely exposed. It also has little pictures showing you where to connect things. (Like the little mouse picture on the mouse port.