Part of preventive maintenance, I guess. To get good gas mileage, protect the engine, and not stress the ignition system to an early failure.
My 1st Gen CRV owner's manual says to change the plugs every 24 months or 24,000 miles, whichever comes first (probably for copper plugs). The NGK platinum plugs I put in them lasted 20,000 miles (10 years).
2007 Kia Sportage manual says change iridiums every 100,000 miles. I had to change 'em at 40,000 miles (8 years).
In both cases, the spark plugs announced the need to replace when the engines ran rough.
I have yet to change my 2012 CRV's factory iridiums as they are only at 30,000 miles and running well.