The fuel injector nozzle has tiny, microscopic holes. Under high pressure, fuel is forced through these holes. This breaks the fuel into a fine mist. Fine atomization ensures the fuel mixes completely with air, resulting in a clean and powerful burn.
Carbon deposits block these tiny nozzle holes. This ruins the spray pattern, turning the mist into a solid stream of fuel. The engine will misfire, run rough, and dump unburned fuel into the exhaust.