The coolant flush is a very important preventative maintenance services. Most vehicle manufactures recommend different flushing intervals, such as every 15k miles to every 100k miles. The coolant life varies greatly depending on which coolant the manufacturer decides to use.
Before we talk about why it is important to change, we must talk about what it is. A coolant that is used in the automotive field is a mixture of antifreeze and water, generally at a 50-50 ratio. Antifreeze lowers the freezing point of the coolant so it doesn't freeze in the winter. It also raises the boiling point. It's main purpose on the vehicle is to provide a means of cooling and maintaining the temperature of the engine, but it has secondary purposes such as providing heat for the passenger compartment or cooling other fluids like oil or transmission fluid. It contains anti-corrosion additives.
Now, as the engine constantly pumps the coolant through the engine, the coolant eventually wears and collects debris such as casting sand. As it breaks down from the pump and from the constant cooling and heating, it eventually fails and will need to be replaced. Periodic changing of the coolant will significantly extend the life of your engine and reduce future expensive repairs. Sometimes if it wasn't changed regularly, you may need to actually flush the entire system with flushing device and a chemical additive. In most cases, a coolant replacement does the job just fine.
This is pretty easy task to accomplish yourself if you have minor mechanical ability. If using a chemical flush, be sure to read and follow all directions on the bottle. Also be sure that you get all of the flush material out.