For single-player use, it is simply a preference. However, many competitive servers ban the use of "X-Ray" packs. While Fullbright is technically different from X-Ray (which makes stone transparent), some server anti-cheat plugins flag it as an unfair advantage. Always check server rules before using it in multiplayer.
In standard Minecraft, light levels dictate how bright a block looks. A cave with no torches is pitch black. A Fullbright pack modifies the game’s internal brightness parameters or the light map textures themselves.