The Vic-20 used two 6522 VIA chips for I/O. I asked why it had two of them but thanks to a comment from Bruce Abbott, I now think that wasn't quite the question I needed to ask.
What I really need to ask is why it needed any at all? For example, the ZX Spectrum had no equivalent chips, and seemed to communicate with its keyboard and peripherals just fine without them. The Vic was supposed to minimize cost. The designers wouldn't have spent money on extra chips for no reason. They must have seen some advantage to be gained by including those chips.
What advantage did the Vic gain from the VIAs, relative to machines like the Spectrum that lacked them?