A glass screen protector isn't the same as a regular screen protector. While a regular screen protector consists of a plastic adhesive, a glass screen protector is a hardened piece of glass. This means the screen protector absorbs the forces exerted on your phone, for example, from a fall. This means it's not the screen of your phone that cracks, but the glass screen protector.
You can then simply peel the glass screen protector off and replace it with a new one. The advantage of this is that you don't need a new screen if you drop your phone; a new glass screen protector is sufficient. Simple, quick, and affordable.