A steering damper is the motorcycling equivalent of an industrial vibration snubber. It allows slow long strokes while arresting quick short ones. Don't get lost on a tangent to the bedroom now (-: The smaller it is, the less effective it will be and/or the harder it will need to work (aka the shorter it will remain effective). It is commonly fitted on light-weight sport/superbikes as well as on quick-steering adventure bikes and sport-tourers with rakes of less than 27 degrees. Never on cruisers. The R18 sports 32.7 degrees of rake, 5.9 inches of trail, and a 68.1-inch wheelbase - proper cruiser parameters. A half-decent steering damper will feature at least three times the cylinder length of that on the R18.
So it is probably a 'judder arrester'. Intended to prevent old men from panicking when they let go of the bars while on cruise control at lower speeds and the front end starts shimmying. The second-gen GL1800s are famous for that at around 40 mph or lower. The most effective cure is to fit a steering brace on those GoldWings.