Why? It’s a little rubber piece for your sight that protects it from things that would scratch it, since the sights is water resistant and such already. Trijicon made a good sight.
This is basically a condom for your sight. To protect it from dirt. A filth prophylactic, as it were.
And I don’t know about you, but my firearm and it’s sights don’t need condoms, since I know how to clean firearms and optical sights.