Right after ordaining Aaron and his sons to be his (and his new temple's) personal priests, God told Aaron one of his major responsibilities as his new priests over Israel was to teach Israel about clean and unclean things, and then judge, and enforce these laws in Israel concerning unclean things.
One of the major reasons for these new laws of purity was since his Prescence would now be in the temple in Israel, all of Israel now had a responsibility to keep his new temple/Prescence from filth/common things so they would not defile his temple and be killed for their defilement:
Leviticus 15:31
“You must keep the Israelites separate from things that make them unclean, so they will not die in their uncleanness for defiling my dwelling place, which is among them.”
And it was not just about his holy temple/Prescence: Since he was holy/clean: free from all filthiness and commonness he also wanted his people/nation to be holy and clean like him: following these laws would keep them clean and holy from the dirtiness all around them:
Leviticus 11:44-45
"I am the Lord your God; consecrate yourselves and be holy, because I am holy. Do not make yourselves unclean by any[thing]...I am the Lord, who brought you up out of Egypt to be your God; therefore, be holy, because I am holy."
So, God began telling the laws of cleansing/purity by defining what is clean and unclean and telling what to do about it to Aaron:
the cleansing laws: