Using a Split Screen for Easy Compositing
Sometimes you may want to do some simple special effects using the same video, just at different times of the shot. Probably the most common use of this is creating your own twin (Think about any version of The Parent Trap), which is so easy with current technology. Lock the camera down, then record yourself (or your actor) at two different times in opposite ends of the frame. Then, drop two copies of the video in your editor (one on top of the other) and split the top layer on one side so the bottom layer shows on the opposite side. Now just adjust each clip to your liking and presto! The twin effect.
This basic technique can also be good for hiding parts of a static shot you don't want to see (like a boom mic poking into the frame), or creating seemingly impossible shots like an empty city street (which I recently did for Episodes 3 and 5 of the web series Midnyte).
Of course you can always use more complex masking, but sometimes the simplest method is also the best.