Two reasons. One, the eggs are triggered to hatch out if they get wet and have a strong light source on them. No light, no hatching. Two, when the infusoria hatches out from the detritus they gather towards the light source. This is usually the water surface in nature. So we need a light above the water. When the Triops hatch out, they move towards the surface and can therefore eat the infusoria which they expect to find there. If the Triops don't get enough to eat in the first 3 days, they very quickly starve to death. A simple Mains table lamp with a 40W-60W bulb a few inches above the water is all that is required.