The great thing about digitally restoring files, is that it is a non-destructive and repeatable process. You start with a digital file, you save it, you work your magic and you end up with the best that you can do. So best practices for restoration have it that you safe the original file. This file is unlikely to be used anywhere and a new template has been created to signal the relevance of an original file.
On the English language Wikipedia, images that are not used are deleted or moved to Commons. This is not always possible for images that are restored. Some images are public domain in the USA but not elsewhere and consequently cannot be uploaded to Commons.
A great example of an original file is this poster for Rome. It is great because it is quite clear that this is how this poster was made available by the Library of Congress. And it allows anyone who thinks he can do better to improve on the existing restoration, a featured picture..
Another reason for keeping the original file is that it prevents a lot of grief. Many people assume that because a file is available under a free license, that they can change the file because it needs an "improvement". Such an "improvement" can be result in a cropped version or a change in the colour balance. While it is technically correct that you can change a file in this way, it is often very controversial. It is very much in the Wiki spirit that you allow for people to make up there own mind what is best.