Modern displays are generally much more robust than they were just a few years ago. In addition, innovations like Gorilla Glass have significantly increased the scratch resistance of mobile phone displays. Nevertheless, there are several good reasons for using a screen protector.
A Short Excursion into Materials Science
The harder the material, the more brittle it becomes (there are only a few exceptions in technical ceramics). If a material is brittle, it can break relatively easily despite its enormous hardness, as a small localized peak load (= force) can cause the display to shatter. The smaller the impacted area, e.g., a phone display falling onto a small sharp stone, the greater the pressure exerted on the display surface (pressure = force per area; the smaller the area of the applied force, the greater the pressure). The greater the applied pressure, the more easily a display that is otherwise quite scratch-resistant can be damaged, resulting in scratches, cracks, or even a break.
Protection against mechanical stress
Display protection films from the manufacturer Neoxum are made from a 0.13 mm thick special polymer (PET) with excellent damping properties that counteract pressure with counterpressure and distribute it over a larger area, so that the applied force is spread out. No scratches or major damage occur on the device display. Modern displays often have scratch-resistant Gorilla Glass. Nevertheless, these hard surfaces are not necessarily scratch-proof.
Micro-scratches
Micro-scratches, caused by keys, glass or stone tabletops, and much more, can manifest as scratches. A predetermined breaking point is created. With the help of a high-quality screen protector, such risks can be avoided by adding an additional protective layer, effectively protecting the actual display from scratches and damage.
Resale value
Nowadays, smartphones are replaced much more frequently than was the case just a few years ago. Some providers even offer a flat rate for mobile phones. It is therefore all the more sensible to protect your valuable electronic device from damage for resale, in order to keep the resale value as high as possible. Every scratch on the display would reduce the sale proceeds.