Using a Fullbright texture pack is a client-side modification, meaning it only changes how the game looks on your screen. It does not give you an actual gameplay advantage like seeing through blocks (X-Ray).
Believe it or not, Fullbright can sometimes improve FPS (Frames Per Second) on lower-end machines. By flattening the lighting engine and removing the need for the GPU to calculate complex dynamic shadows and light updates, the game runs smoother for some configurations. fullbright texture pack 1.12.2
Note: While texture packs simply alter visual files on your screen, always check your specific multiplayer server rules regarding brightness modifications to ensure compliance. How to Download and Install Fullbright for 1.12.2 Using a Fullbright texture pack is a client-side