If I gave you a sheet of 8.5”x 11” paper and told you to fill it with a drawing you, would most likely be able to do it. But I imagine you would only ever go up to the edge of the page and never draw off of it. The edge of the page would be a very strong border holding you in. Plus, why draw off the page anyway? You filled the page just fine by drawing up to the edge and not off it. And you’re right. What’s the point?
But writers on the other hand must write off the page if they want to create a world in which their story is set. Otherwise the story is an isolated string of events that happen in a very contained environment.
Of course words shouldn’t flow off the page and get cut off. There should be margins and the text should be legible. Writing off the page is creating the content that never makes it into the story, but is still there.
What does it mean to write off of the page? It means making connections to something larger than the story being told. It is creating the world in which the story happens. Why? Because it makes the story more credible and more interesting.
Take J.R.R. Tolkien for example. In his story, Lord of the Rings, there are fragments of hundreds of other stories that pass through his but are never told. Why does that happen? Because he has created a whole world and it naturally seeps in. A story is not a straight line that moves from one point to another, its more like a rope made of hundreds of threads that are tangled and weaved with many loose frayed ends. It wraps around and maneuvers a landscape, never in a straight line.
Or, it’s like a picture that runs off a page.