This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
While it is true that Struts tags involving a URL do perform URL rewriting, the main purpose of the code you've shown us is to create and use a URL for a specific action. If you're using the default suffix of ".action", the URL would translate to something like:
It's considered a better practice to use this method rather than specifying the ".action" because even though the default suffix is .action, you could set it to something else. The code you have shown us will work regardless of what the suffix is.