Supplementing

Formatting XSLT is designed to add formatting (XSL-FO) information to documents, while stripping out the original document structure, which doesn't help with formatting. CSS avoids an explicit 'transformation' model, but definitely adds formatting information through its annotation approach.
Pulling from related resources Using keys (think SKUs) and other information, you can start from a base set of information and add additional information (like whether an item is in stock or not, whether a credit card is declined, etc.) to the document, before passing it to a different processing.
Adding routing information If you send XML documents from place to place, you may want to know where they originated. You can add routing information to your documents, even specifying a path back. This requires all parts of the route to be interested in maintaining such information, however.