Abrasion occurs when two surfaces rub together or when hard particles slide across a metal component. In car engines, dirt in the oil acts like sandpaper. It wears down bearings, piston rings, and cylinder walls quickly. Keeping your oil clean prevents this damage.
Mechanical seals and belts also suffer from abrasion. Road grit gets under belts and grinds against pulleys. This ruins the rubber and causes belt noise. Regular washing and filter changes keep abrasive dirt out of critical systems.