Question

why is it important for granaries to be clean and dry?

Answer 1

Because if these are clean the grain would not get dirty which is good for it.
and moisture also affects it

Answer 2

Granaries are places we store grains. In order to keep the grains fresh and clean, the place should be clean where it is kept.

So it is important to keep the granaries clean and dry because moisture can help grow microbes and fungus that can ruin the storage. To keep it dry and safe, it should have a sunlight exposure or sprayed with environmental and food friendly sprays.