A system unit is a piece of hardware that houses other components, especially in a computer. Inside the system, the unit resides the Central Processing Unit (CPU). The CPU performs all calculations, executes all commands, and stores data for future programs to use. In order words, without a system unit, you would not have any computing power.
There are three core parts to every computer: monitor, monitor stand, and the different large tower which contains all of your wires and connections; this machine itself is called the “System Unit”. Know that even if there’s no PC monitor you still need one: an LCD monitor or an LED monitor.