I've seen that several times too, but I've never thought about it until you asked that. I thought it was dependant on how long your code and output is, but I doubt that is right. I'm not sure how to do that myself, but can see the benefits of doing that so you can see the code and output at the same time rather than scrolling down the page.
Troll...and I know who, the troll from the jobs thread. He is the only person I've made mad enough to do it, but he doesn't realize the more he does it the quicker he will be banned because my posts are on topic and nothing wrong with them.