Iridium plugs offer a couple advantages over the standard copper core NGK plugs.
First off, the fine wire center electrode produces a more exposed spark that helps ignite combustion faster, and the sharp tip makes it easier to produce spark at a lower voltage, so that reduces wear on your entire secondary electrical system (IE coils and wires) as well as your starter motor and battery from less cranking during engine start.
Second, over time they will maintain their gap better. This means that your gas mileage and performance will remain more constant than with the standard NGK's, which starts to gradually drop off after about 7K-10K miles or so. If you looked at the curve for gas mileage and performance over the life of the plug, you would see that the iriduim plug curve is much more flat, where the copper core NGK begins to drop off a lot sooner.
And the third benefit is the obvious one, that they simply last longer and don't need to be changed as often. However, the increased cost has to be factored into that.
Also, don't be fooled into thinking they won't ever wear out. The ground electrode on them is still made from a nickel alloy, so it still wears out at the normal rate. Only the center electrode is iridium. According to NGK, their main purpose is for increased performance produced from the fine wire electrode, not longevity. Plugs that are truly made for longevity are called "dual precious" as both the center and grounding electrodes are made from precious metal to reduce wear on both. But they are hard to find and extremely expensive.
By the way, the GL1800 uses a wasted spark system which increases wear on the plugs and wires significantly. I'd sure like to see Honda upgrade the next Wing to a more modern system with individual coils on each plug. Most folks never change the plug wires on the GL1800 because to do so I believe you also have to replace the coils, as they are molded into them - and you have to remove the top shelter, gas tank, and airbox to get to them. I suspect there are a lot of GL1800's out there running around with worn out and leaky wires on them as a result. So from this standpoint, iridiums are a good idea, because they will fire off at a lower voltage and thus will reduce the strain on the wires by keeping the voltage down, so your plug wires and coils should last longer.