The rules – laws – on bodily discharges.
Water must have been scarce, and water for bathing and for laundry must have been a luxury in the Old Testament. Something I easily take for granted. Also, women have to go to the temple to sacrifice at least once a month – that must have been expensive.
The lesson for me here I think is, not all things that causes us to be unclean are deliberate. Some things are just part of our human nature, something we cannot help. For those things, we still need cleansing to be able to approach God.