An ISO file (ISO image) is an archive file that contains the contents of an optical disc, such as a CD or DVD. For macOS, an ISO file would contain a bootable copy of the macOS installation media. This can be particularly useful for virtualization software users who want to install macOS on a virtual machine or for those who need to create bootable installers.
To understand why sites like TechRechard are popular, one must understand the technical hurdles of macOS installation methods. techrechard macos iso