I think they want to go back to prior levels without having to make them again as a new level. I think there are a few mods that support this, the mega compiler one sounds like it does anyway. Must admit I would find it useful to be able to change some basic scripts and entities and return to a level under different circumstances without having to basically clone the level with the changes and call it a new name for instance.
Of course the only thing I don't like is the extra space it could take saving the same level twice. If it didn't actually take up new space it wouldn't be an issue.
http://s6.bitefight.org/c.php?uid=103081