"You need not fear. Cybermen will remove fear. Cybermen will remove sex, and class, and colour, and creed. You will become identical. You will become like us."

— CyberLeader 1, Doctor Who, "Doomsday"

story involves characters who don't want to surrender their fleshy bodies. And that can be handy to know for a villain. This is normally done by dictator types. This can be to suppress rebellion, to remove individuality, or to create a robot army - or a combination of the above.
Some devices that inflict roboticisation use nanomachines, resulting in graphic scenes of the person being slowly mechanized while screaming in pain as they are being rebuilt into machines.