For the most part, in my experience (which only covers one jurisdiction) Courts do what they think is fair with regards to splitting property for a divorce.
It seems that the most common scenario with pre-nups is this: A wealthy (or potentially wealthy) man asks his fiance to sign one. She does because she loves him and doesn't want him for his money, she thinks it won't matter because they won't ever divorce. 10-20 years pass by and wealthy man is ready to trade-up for a younger model. The woman who raised his children, stayed home, didn't get an education (often supported hubby while he got his M.D. or J.D.), has never worked a day in her life, finds that when hubby decides to move on to greener pastures, she is faced with the pre-nup she signed back when she was naive and idealistic and thought his promise meant something.