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 the glass screen protector won't crack, but the screen itself. You can then simply peel the glass screen protector off and replace it.
The advantage of this is that you don't need a new screen if you drop your phone; a new glass screen protector is all it takes. Simple, quick, and affordable.