Amongst the rules for the priests was a limitation on whom they could marry. The reason was so they could:
Ezekiel 44:23 … teach my people the difference between what is holy and what is common, what is ceremonially clean and unclean.
This need to separate the common from the holy was explained again and again. It was something the first Israel nation either didn’t understand, or forgot, and so there was a need to repeat it the second time around.