Numbers 19:15 And every open vessel, which hath no covering bound upon it, is unclean.
God told His people that a vessel that was not covered would be contaminated if death or sickness took place near it. Covering the dish would save the contents. An open dish would be contaminated from the germs in the air.
What a wonderful God. What a wonderful book. Moses knew that by divine inspiration long before any scientist ever understood it!