A ROM, or read-only memory, is a form of computer data storage that can be written to but cannot be erased. Modern phones and tablets store the operating system and applications in situ on the original read-only memory sold by the manufacturer of the device. The user can install some applications from third parties on this ROM/ROEM, but not others.
Since many malware creators use loopholes in commercial software to get onto your machine – read-only memory (ROM) such as an object code program (to make booting up more efficient by “hiding” key parts of the program). But if you want to protect against it you need writeable memory, like a hard drive or RAM which is usually expensive