Shielded Twisted Pair noise immunity
A shield usually makes a pretty good protector against electric field disturbances (half the story) but, unfortunately, makes a pretty poor protection against magnetically coupled noise (the other half of the story). 

Twisting the conductors in a pair mainly ensures that induced voltages (due to external magnetic disturbances) are equalized on both conductors thus, their effect can be defeated with a properly designed balanced receiver circuit.