Items might be second-hand because they gave the previous owner problems.
There are electrical items (example, wrist straps) that have the advice, "Connect grounding screw to earth, by means of a wire to a radiator, metal plumbing, etc." However with plastic pipes in wide use, we cannot be sure if there is a conductive path to earth.
So the previous owner may have had trouble with the unit. Maybe it did not dispel static charge like it should, or perhaps he got a shock from soldering an appliance which was inadvertently plugged into mains.
Perhaps the previous owner replaced the power cord, so the unit would be compatible with receptacles in his home.
Etc.
Does the unit have a grounding screw at the back?