Why computer is dummy device?

Answer 1

A "dumb terminal" is a type of computer which is not designed to be used by itself. It is just a data entry device, much like a computer keyboard, but with a display. It connects to a system using an ordinary computer or a mainframe. It is called "dumb" since it cannot run software and requires another computer to be of use.

There are other types of "dummy devices." The term can be used for computer components that don't have their own controller. They either plug into a controller card, or a device of a similar type that does have a controller. Some of the early Apple floppy drives were like that. There were expensive drives with controller circuitry, and lesser expensive drives without it and which required being connected to a drive with the controller. That is also how the terms "master" and "slave" were coined in regards to computer components.

There is also the "null" device, which is technically a "dummy" device since it is not real hardware and doesn't do anything. It is a virtual device in an operating system, notably MS-DOS, which is used to silence the output of program. You could redirect the output to the null device.