There's an interesting post on Brian Jones's blog today about how rsids work in Open XML documents.
The use of rsids provides a simple and flexible mechanism for merging changes two users make to the same document, and if you've looked inside many Open XML documents you've seen rsdis all over the place. They're used to identify "batches" of changes to text, styles, properties, and many other elements.
If you're writing code that modifies existing documents, you may want to implement rsids. That will allow users to merge changes made by your software as intuitively as they can merge changes made from within Word or Office, and your modifications won't have unexpected behavior for users who rely on Word's merge-changes functionality.
You can design your own software to handle rsids exactly the same way that Office does, since the details are all documented in the spec. Brian's post will help you get started.