The reason is that along with volume labels and the data, if you want to make bootable USB or DVD, the boot information will also need to be included. However, unlike other types of files and content, it is not possible to simply copy and paste the contents of an ISO image.
The most common way in which ISO files are used is to create bootable disk for the purpose of software installation or repairs. In technical world, image file is referred to an ISO archive that compress all kinds of files and folders in one place.
