Why is a ketubah needed for a man and woman to live together?

Answer

One reason is that the woman should be protected financially in case of losing her husband through death or divorce.

Another reason is that it gives the man reason to "think twice" before making any rash decision to divorce, since there is an extensive "get" procedure, and also the ketubah makes it financially burdensome for him. This should encourage the couple to seek counselling and work it out.