Mushrooms that glow in the dark, vines that shine, glass blocks that glow softly and can be used as crystals… light source blocks are few and far between in Minecraft. We only have sea lamps, glowstone and redstone lamps.

But wouldn't it be nice if you walked into a cave of pure darkness and little blue-white dots of light were everywhere? Not enough to light the place up, but enough to give you a feel for how big it is? Or to be traveling in the darkness of the nether and to see red mushrooms glowing on ledges instead of just lava and fire everywhere?

In complex cave systems, biologically produced light sources are a popular way of attracting prey (usually insects), and would add a lot of atmosphere to the sometimes very plain caves of minecraft.