So that you can edit and see what your page would look like, without other people seeing the changes.
Nice to have, maybe combine with a 'revision history'.
Then you can see past/future versions
(you have to store two versions anyway, so why not many)
I wouldn't do this for edges, just for properties like body, summary, etc.
Commented on Google Code by *profile.url***
For the record, mod backup now provides the revision history feature.
So revision history is only visible when mod_backup is enabled? I think it should be a separate module.