Cardboard on top of soil, why?

Because they read & watch get rich quick or make killer bud. U need air moving through ur root zone. And how it does tat is by taking air in while it dries from the top down If u worried about pests. You can use clay pebbles and they won't be able 2lay a thing Good luck. They will not help keep away gnats. A 1 inch layer of pebbles is needed not papper. Ppl use a white sheet around their DWC tanks as it reflects light back keeping the sub (water temp lower) tat could be partly y. But in no way would it function like tat with soil

Because they believe ghetto science.

It's for moisture retention mainly, but also will help with fungus gnats and keep algae from forming on wet soil surfaces.

I grow in soil never put cardboard over it. My guess would be they use it as way to control gnats.They could also be using it as away to slow down evaporation of the soil.

I assume its to try a stop light penetrating into the soil similar to how people do the same thing in NFT or DWC systems if there is alot of humidity and light algea and mold will form I suppose putting cardboard there would try and stop this but I think theres a more sutible material for this other than cardboard mylar would be better