Isolators can galvanically isolate two communication lines. This comes in handy in cases when you interconnect two devices powered from two different power supplies and there is a danger of a ground loops occuring. Ground loop can be created by improperly installed or designed system, when parts of that system are powered from different power sources on a different electrical potential. Balancing currents resulting from those different potentials can damage your equipment very easily. Galvanic isolators prevent ground loops and completely protect connected devices from damage.