Why application software and system software needs to be compatible?

There are lots of types of computers and different software is written for different kinds of computer. If you have software that is not designed to work on your computer and your operating system, it is no use. Applications need an operating system in order to be able to work. If the application is designed to work on one operating system then it cannot work on others. So if you have a PC, buying Mac software for it is no use. If you have a Mac, buying PC software is no good. Different versions of the same software are brought out for different computers, so you need to have the right one for your computer and operating system.