What could sound better than getting a good flushing out? "Hey bud..your coolant is looking a little muddy….could use a good flushing." Unless your manufacturer recommends a flushing, it's a waste of your hard earned money. The coolant, which can be topped up if need be, is designed in some cases to stay in the car for five years or 150,000 miles.
And you have to be mighty careful about topping it up. Let's say you have a brand of coolant like Dex-Cool, which is orange in color. This is a long-lasting coolant preferred by companies that buy cars in fleets, like state governments and pharmaceutical companies. If someone adds green coolant, which ethylene glycol based, your orange coolant will be degraded from lasting 150,000 miles to lasting about 30,000 miles. If you bought your car used, it could have been a fleet car.
If you start out with green anti-freeze, then change it every 30,000 miles. An actual flush of the system is only recommended every 50,000-60,000 miles.