I bought a tempered glass screen protector for my Samsung A5. About £3. Dropped my phone many times, landing screen down. After 2 years when my contract was up, I removed the protector. It was so battered it almost crumbled away in my hands. Revealed underneath was a completely scratch free and like new phone screen. I would say yes they do prevent phone screens from cracking.. Tempered glass remember!!! Just bought one for my new Samsung Galaxy Note 8.