Because you reduce the chance of your engine/turbo dying early(ier). If you change cars every 60k miles I agree it doesn't make sense but if you are planning to keep it for a while why not make reduce the risk?Lots of people needlessly changing perfectly good oil. I'm interested to know why 5K seems to be a 'good' time to change it?
Mainly for diesel owners. I would think diesels with turbos would benefit from oil changes before the recommended 20k from some manufacturers.