So yes, the Tableau packaged workbook (.twbx) file is actually a zipfile. This ZIP file contains the workbook, but also all supporting files that are used inside the workbook. Think about:
When will you use this? And how do you get all this information out of the Tableau workbook? Here is an example on how I found out I needed to know this.
A colleague send me a workbook with a standard dashboard template for our company. I needed this template with all the images and custom shapes for my own dashboard. Instead of asking him to send me all files separately, I did the following:
Everything in the workbook send to you is now in the new folder and ready to be used by you 😀