So far Ubuntu (and derivatives) has been delivering ISO images to be burned onto optical media. Then there's a tool (usb-creator) to "burn" those ISOs onto USB drives. I could be wrong, but I think that nowadays the majority of users don't use optical media to install Ubuntu (as well as other OSs) also because optical drives are not any more standard.
A "USB image" is usually "burnt" with a standard dd which is readily available within whatever OS you already have. While if you are not coming from Ubuntu it's likely your conversion tool is different from Ubuntu's and it won't work flawlessly.
What is the advantage for Ubuntu to keep the ISO instead of a "dd-able" image?