For example you want to have ric olie do something in your edit but you cant have him because of the lack of footage. so you can take some shots from movies or comics that includes him and place dilaogue or whatever you want without caring about lipsync or it looks good.
http://www.youtube.com/watch?v=6Uv-9mJCTkk
Now this is the opening scene but you can get the idea when something is going to happen instead of live action footage you have comic images doing stuff :)
-Angel