Hard coating are usually used on eyeglasses with plastic lenses. You know , today most of eyeglasses lenses are made of plastic lenses. The plastic lenses is lighter and safer than other glass lenses. But the plastic easy to get scratches. So, we need hard coatings which can used to protect the plastic lenses. They can toughen the surface greatly.
The hard coating is used to prevent your lenses from being scratched. If the lenses of your glasses are made by plastic, you'd better coat them with hard coating so that they will be scratch resistance. But if the lenses are made by glass, you don't need to coat them because glasses are scratch resistance.