The priests had to avoid contact with death. They could not enter a place where a dead body was present. They also could not follow certain mourning customs, such as shaving their heads or letting their hair grow wild.
They were not allowed to drink wine before entering the inner courtyard. This rule helped prevent serious mistakes that could lead to death (see Leviticus 10:9).
The priests also had rules about marriage. They could marry only a virgin from Israel or the widow of a priest. These limits helped protect the purity of the priestly family line.
In all these ways, the priests were to show the people the clear difference between what was holy and what was common, and between what was clean and what was unclean.