Aaron and his sons are ordained: Leviticus 8–10
The point of these complex rituals—sacrifices, blood, burnt offerings, strict rules for priests (including not drinking)—is for Aaron and his descendants to learn the different between the everyday and the sacred:
Leviticus 10:10-11 You must distinguish between what is sacred and what is common, between what is ceremonially unclean and what is clean. And you must teach the Israelites all the decrees that the Lord has given them through Moses.
This is a constant theme throughout Leviticus: the idea that God is holy, and therefore the Israelites were to live as a holy people who were different to the peoples around them.