People can touch their nose with eyes closed because they sense where their limbs are. Robots need the same internal sense, called proprioception, provided by sensors in each joint that report angle and movement, so the machine knows its own posture without seeing it.
This internal picture is the basis for smooth, controlled motion and for catching faults, since a gap between commanded and actual position signals a problem. Without it a robot would have to watch itself constantly, which is neither fast nor reliable enough for real work.