The original Mac was built before there were Microcontrollers as we know them today, and a lot of the I/O circuitry which has found its way onto system chips just wasn't put on a chip with the CPU back then.

If you're familiar with modern microcontrollers, then a good analogy is that the VIA (versatile interface A) is like an addressable microcontroller port. One accesses it at certain addresses in the address space, and it provides the kind of port services we've come to expect from microcontroller ports on a modern chip.