When you have a lot of different kinds of documents and different people accessing the documents for different reasons -- for example, new employees in a certain department -- having notes on the folders helps end users decide where they should be filing their company documents.
The notes should be editable by the person who created the note. This is so they can update the purpose of the folder and any additional documents or explanations necessary. Also, if they happen to make a spelling error, or decide later that a note is no longer necessary, they should be able to edit or delete a note.