Why is operating system a mandatory software?
There are different hardwares. Many more hardwares are about to come. Its not possible for each hardware to support each type of software. But the possible things is there should be an universal software which should be adopted by each and every hardware.
For example Windows 10 operating system.
Any processor, any keyboard, any mouse, any printer, or we can say any hardware. Vendor have to wory only about that, that hardware should be able to accept the perticular operating system. And rest is left on operating systems.
Because operating systems are build as it should be able to support all other softwares and hardwares. And other softwares are build as so they should be accepted by perticular hardware.
Thats are operating system is a madatory software,