There are many designs of small 8-bit computers that are based around a single pcb or single monolithic design. While a single pcb, self contained computer looks really cool and trendy, it is a once only build. You don't have the flexibility to upgrade, change or expand the system when you want to try something new or add extra functionality.
The goal of the Whizz80 is to allow the design to grow and expand as I develop it (it is iterative process as I learn, so mistakes would be made). Because of this I am deciding to use a modular approach and have divided up the computer into 2 distinct core modules. The System Core and the I/O Core.
The 2 cores are connected to each other via an I/O Bridge. You can run the Whizz80 without the I/O Core and I/O Bridge, but it wouldn't be a very exciting computer (and the only way to interface with it would be directly on the system bus)