You must keep the Israelites separate from things that make them unclean

 Leviticus 15:19-33

When a woman has her regular flow of blood, the impurity of her monthly period will last seven days, and anyone who touches her will be unclean till evening. Anything she lies on during her period will be unclean, and anything she sits on will be unclean. Anyone who touches her bed will be unclean; they must wash their clothes and bathe with water, and they will be unclean till evening.

If a man has sexual relations with her and her monthly flow touches him, he will be unclean for seven days; any bed he lies on will be unclean.

When a woman has a discharge of blood for many days at a time other than her monthly period or has a discharge that continues beyond her period, she will be unclean as long as she has the discharge, just as in the days of her period. 

When she is cleansed from her discharge, she must count off seven days, and after that she will be ceremonially clean. On the eighth day she must take two doves or two young pigeons and bring them to the priest at the entrance to the tent of meeting. The priest is to sacrifice one for a sin offering and the other for a burnt offering. In this way he will make atonement for her before the Lord for the uncleanness of her discharge.

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.

Neither was sinful in itself and there was no need for sacrifice. In later Judaism, only the rules concerning menstruation continued, effectively barring women from worship or contact with men at that time. In Christ, these restrictions no longer apply; no woman should be afraid to read the Bible and join in worship at such a time.

Why?

In that generation of the old testament, Tabernacleis holy place, God asked his people have to clean to worship Him. Now after Jesus came, there is no restriction about this, but let us know that we belong to the Lord, we have to keep us from the world and keep us clean, separate from this world, away from sins.

The regular flow of blood is a physical reaction to a woman, it is not sin but unclean, for her healthy, she needs to take rest at home.

Discharge is a disease of woman, it needs to be cured by a doctor, Jesus has healed the woman who was discharged. Even this woman should not touch others, but she believes Jesus could heal her so she touched Jesus.

Before Israelite enter Cannan, God warning them to keep away from their dwelling place, or they will die in their uncleanness.

How?

Keep us from unclean and sins among our living place, but to be a light to shine them.