While using iMovie, I want to easily create snapshots of my project so that I can easily go back in time and grab things that I need.
For example, if I'm making large scale changes, and I accidentally messed up a clip or a nice transition, I want to be able to go back in time to grab the version of the clip before it was messed up.
In its default state, iMovie has you working on one single version of a project, and your only way to go back in time is by using the undo feature. However, this is not preserved between app launches, and probably has a limit.
I know I could copy an entire library, but that would take up a ton of disk space as all the videos would be copied too.