Your coolant is what helps keep your vehicle from overheating. It, just like your vehicle's oil, can break down and cause sludge. This wears down your vehicle and could cause it to overheat. This is not good. A coolant flush allows you to get rid of all of that broken down coolant and replace it with clean coolant. It also allows you to clean out a bit of that broken down coolant, preventing it from causing further damage to your engine.