When you create a Microsoft Word file, you may think that all the information you’re giving is what you type into it and keep in the final version. If you’re seriously concerned about confidentiality, you can’t count on that. A file’s metadata can include information about its source and history which you never realized was there. Redaction may not remove all the information it’s supposed to chop out.
When you or somebody else installed Word on your computer, you were asked to enter information about yourself. It gets put into every file you create. If multiple people edit a document, the information on all of them gets into the metadata. Most people don’t mind, but in some cases it could be revealing too much information. If you entered gibberish or silly comments, they go into your documents.