Explain why unlike the operating system,the computer can work without an application software?

That depends on what you mean by "work". In the XT computer, the operating system only loaded a program. Then it turned the entire computer over to the program. The program had to figure out what kind of a video card it had, what kind of sound card, what kind of modem, and what to do with every keystroke and mouse click. Then came the Intel 386 processor and Windows operating system, and modern computing became possible. The 386 processor had the ability to pretend to be hundreds of computers running at the same time, and that gave Windows the ability to run hundreds of services, and monitor them all for correct operation. The only one of which you are aware is the clock/calendar. So the computer is doing plenty of work with out an app, but your question doesn't mean anything because it is so far removed from what the computer actually does.