The .OLD extension is sometimes used to indicate that the file is an old version, and you'll often find the newer version in the same location.

For example, in a folder there was originally a text document named mytextdocument.txt which was superseded by a newer version. As you can't have two files with the same name in the same location, the extension can be changed from .txt to .old

We now have two files in the same location, mytextdocument.txt and mytextdocument.old