On the original KIM-1, RW0 (pin 17), RW1 (pin Z), RW2 (pin X), and RW3 (pin V) are the row currently being scanned on the keyboard. (Only RW0, RW1, and RW2 are used by the actual keyboard it looks like.) They go high based on the decoded binary number sent to U45, a 74145 with ten lines out.)
Along with PA0 through PA6, you could, in theory, build an external keyboard. I'd have to dig into the ROM code, but if RW3 isn't used on the KIM motherboard, it seems like it could be used to be something like an Atari style joystick or a few special purpose keys.