Computer System = Hardware (Physical machine) + Operating System (Software).
They serve as abstraction over hardware, letting users to control computer systems without too much technical knowledge. They should be easy to use.
Abstraction over hardware.
It can be explained as generalization of hardware (and hiding unneccessary details from user to make their lives simpler). It does not matter what exact hardware (graphic card, soundcard etc..) You have, operating system abstracts over it.
You have the same user interface, it does not depend on which model of hardware you have. From user perspective it's only better or worse sound or graphics or computation speed (or other things). Users do not need to know how it works, how software handles hardware, how drivers work and so on... All that matters is performance gained.
Hacking software and understanding how machine works is for hackers. Anyone can train to be hacker, but it's paid with precious years of life.
Operating Systems should make Hardware secure, fast, and easy to use. Users should not waste years of their lives connecting cables and solving problems to print simple document or whatever they need to do.