The oil inside your shocks brakes down and no longer dampens or protects your parts properly. Failure to change this can result in costly shock repairs. Most shocks with over 1500 miles are not working properly, but the rider has adapted to them not working properly.  Most shocks don't just go out, it's a slow process over hundreds of miles. Putting a few dollars into your shocks every 1500 miles, will keep your machine working in good shape and handling good.