Why Marriage, Anyway?

I teach a Marriage and Family Living Class to high school juniors and seniors and do couple marriage workshops with my husband. From time to time I will have students who are already living with a partner, a couple even buying a house with their partners.
When we discuss marriage in class, I hear them say -- "I'm not getting married until I have lived with the person for a few years first." When I ask why they choose cohabitation first, they often reply, "I don't want to go through a divorce like my parents did."

Sometimes I will hear middle aged friends say, "Marriage hardly ever happens." or "That might be okay for the younger ones, but not for people like me - I can't afford another divorce."

What are the benefits of marriage to a man? to a woman? to the children?

What is it about marriage that is different than cohabitation?

What's commitment got to do with it?