Go to the Projects tab in Xcode Organizer and select your project. You should see all the pictures taken. Select the snapshot you want to restore and select Export Snapshot to save it to disk. You export to disk your "restored" version of your project.
I agree that systems like SVN and Git are far superior to Xcode's snapshot mechanism for version control. However, the snapshot mechanism can be very useful. For example, I just made changes to the project. Before making changes, Xcode asked me if I want to take a picture. After the change, it turned out that I did not want to make this change, and therefore I used this mechanism to restore the project to its previous state.