Notes are extremely useful, on both files and folders -- especially for people just beginning to use the document repository for the first time. Some of the documents, such as Adobe design files, do not have previews, so it would be useful to be able to describe the use and purpose of a file. Also, for documents used for specific purposes, a detailed note on its allowed usage within an organization is extremely helpful. Notes are also useful for an ongoing log by those who are editing the file.
These notes must be editable by the person who created them. When you type in a note, you could make a spelling error, or you could want to reformat the text of a note. You could also wish to change what you have written, or update the information.
This is a requirement of our organization for managing documents - being able to log notes on each document when necessary. So perhaps it is an important function for other companies that manage many documents, in various languages.