Presume the guys asking about screen protectors on 3d printers don’t have experience with resin. Resin printers use a LED screen under the resin vat. It passes through UV light to cure the resin to make the form.
If you have a cock up with the resin, or other mishap you can trash the LED screen, which is not a cheap part to replace, approx £80-100. To help mitigate this you have a screen protector.
My machine is an elegoo mars 4 ultra. It ships with a screen protector already fitted. I have managed to crack it due to having residue in the resin tank I didn’t spot, which then when the build plate zero’d cracked the protector, but the underneath LED is fine. In fact I’ve been using it with a crack for a while and as long as I avoided that area its worked ok Now the crack has spread and the protector needs replacing.
Elegoo for some very strange reason do not sell replacements for what is a consumable part, well, I suppose it is only a consumable if you have a cockup. No cockups it would probably outlive the rest of the machine
you can use it without a protector, but if you have a resin spill or the like it is probably time for a new one.