Well let's say that you are the director of a play. All your actors learn all their lines by heart. They get their voice inflections down perfectly. They are great with their facial expressions and body movements. Now comes the opening night and the curtain rises but no one has a clue where to stand on the stage. They all stand around flat-footed because no one told them their cues. When they do move, they are bumping into each other or out of position when someone tries to pass them a prop. It is a total fiasco.
If you don't practice O&D that's exactly what you would see.