What is the purpose of an open relationship/marriage?

I don't believe in open marriage but, somepeople do want a life partner. They want to raise children with one person who will work with them to maintain a household, be that daily source of commitment and comfort, but they still want to have sex sometimes with another person. For some people in an open marriage they rarely have sex outside of their marriage. For some people they are bisexual and dont want to totally give up the other sex. Some people have different sex drives but they dont want to divorce over it. Some people never thought that they could or would settle down but their partner told them that open marriage was ok. There are many scenarios. Lots of them just like to have variety of sex partners.