"Marriage is a contract" isn't just a catchphrase, it's literally a contract and serves exactly the same purpose as a business contract.
See how this statement sounds: "If 2 businesses are going to keep doing business together, a piece of paper telling them officially doing business together doesn't make any difference and if 2 business are going to break their engagements, some paper's likely not going to stop it"
In a sense everything above is true, but yet, a businesses wouldn't enter an important business deal without a contract.
Marriage can be seen similarly, when you sign the contract you are doing so with expectations : "this person makes me happy, I'm ok combining forces to go further"/"this business partnership makes me increase my profits, I'm ok doing this deal with them"
Both marriages and business deal can go sour. People change, business constraints change. Getting the right provisions in a contract makes sense in both cases.
Both people getting married and businesses striking a deal don't enter the contract with the hope that they will have to break the contract.
Both know they can break the contract if needed, with effect somewhat known.