The reason to use them is their service life.
Yes they are expensive, here in Australia they are about 3x the cost of a standard plug but they get more than 3x the life of a standard plug.
I have a mk 3 lenans with over 330,000km on it, I have had both cylinders plug threads fail over the years and spit hot active plugs against my legs, in traffic, utterly wonderful.
So less replacing equals less stress on plug threads and less failures.
I recenly did a service on my youngest sons much neglected hyundai ( hes inherited his mothers lack of mechanical sympathy gene) the plugs had never been changed in 75,000 km, so I changed them, what got me was they all gapped within spec and were a good colour and If I hadnt already bought replacements were good enought to go back in.
So its up to you what you do, but iridiums are a better plug for longevity.