Leviticus 13-16 seems obsessed with how leprosy, blood, and semen make people ceremonially unclean. To modern readers, all the details about what to do if you happen to touch blood, or be menstruating, or have an emission of semen, etc., seem crazy. But to the Israelites, this was actually helpful information.
Leviticus 15:31 explains why. The Lord tells Moses:
“Thus you shall keep the people of Israel separate from their uncleanness, lest they die in their uncleanness by wdefiling my tabernacle that is in their midst.”
It was a matter of life and death that the Israelites not enter the presence of God in an unclean state, so the purity laws in Leviticus are meant to help the Israelites know when they are unclean, and to know how to become clean again. Note that “clean” and “unclean” are not moral categories–good people would become ritually unclean all the time.
When Jesus came along, he explained (see today’s reading in Mark 7:1-23–great timing!) that though the ancient people thought that external circumstances could make a person unclean, in fact it is what comes out of a person’s heart-internal circumstances–that make a person unclean. So, we no longer concern ourselves with ritual uncleanness, because Jesus’s death (the ultimate sacrifice) makes all who trust in him clean before God.