Burning CD images to Bootable USB
The Windows iso image needs to be converted to a Read/Write format so the "Boot Camp Support Software" can be added to the USB flash drive.

Say, for Example, you wanted to use the Boot Camp Assistant to create a USB flash drive for the purposes of installing Windows 7. Apple lists the steps at the link "Perform a new install of Windows 7". Step 11 requires using the Finder application to manually copy the "Boot Camp Support Software" to the USB flash drive. Of course, for newer versions of Windows, the Boot Camp Assistant does this step for you.