Unsprung weight is one of the few issues where the goal is absolute elimination: it's not possible, but anything you can do is an improvement (even if minimal or not cost effective).
Something it does do in a more practical waye is to disconnect the spring rate on a street car from devoting too much priority to forcing the wheel down after a bump - a big heavy wheel needs a big spring or it remains air-born after the bump, loses traction and upsets the car.
If the wheel weighed nothing (100% sprung) the spring would only have to hold the car up, and the ride quality would be much better - the wheel could dance over bumps without the chassis moving at all, and this is a big advantage to IRS.
