Why were the priests astronomers?

In ancient times, priests were not only mediators of the gods, but also possessed certain knowledge. If we take ancient Egypt, then here the priests knew not only astronomy, but also medicine, as well as chemistry. Which, in fact, made them very valuable. So the priests were responsible for a very important task – the burial of the body of the deceased pharaoh. It was they who, thanks to their knowledge in chemistry, were engaged in the mummification of the body of the pharaoh.