What is CMD [ Command prompt ] ?

I assume you're talking about cmd.exe on Windows. The first operating system Microsoft released on PCs was MS-DOS. The entire user interface for it was just like what you see when you run cmd.exe. For 10 years after Microsoft started producing Windows, DOS applications were still popular. Windows ran as an application on top of DOS. At first, people used to boot into DOS, and then run win.exe to get into Windows, if they needed it. At some point it became possible to run DOS apps. inside of Windows, using the Command Window. Windows would simulate the operating environment of DOS so that DOS applications would think they had the machine all to themselves. This continued through the 1990s, and into the early 2000s. Eventually, Microsoft stopped supporting DOS applications inside the Command Window, and turned it into a 32-bit console environment. It was possible to write programs that would only run inside a command window, but they would not run on the old MS-DOS.

System administrators still prefer to have a command line to manage systems, and to run scripts for doing the same. Even in consumer environments on Windows,  there are still some things that can only be accomplished using command line tools, since the Windows environment does not offer a graphical tool to do the same things.