Certain agreements MUST be in writing, in order to satisfy the Statute of Frauds. Other agreements do not NEED to be in writing. But in most cases, it is advisable. This is not your grandfather's business world, where people were as good as their word, and where a handshake was respected and sufficient. In this day and time, commerce has become much more complicated, and less concern is given to honesty and decency, and more concern is given to profit. This is not a positive change, but it is reality.
In situations where reasonable minds can differ as to what the agreement was, having the principal terms in writing just makes good sense. If there is ever a disagreement and it winds up in court, it is a lot easier to protect your interests by relying on a written document than on trying to piece together the terms by the subjective testimony of opposing sides.
It probably goes without saying, but most contracts are best left to the attorneys to draft.
*** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state.
It is always a good idea, particularly in the case of business contracts, to have something in writing so as not to depend on "he said, she said."
That being said, the requirements for a contract/obligation in Louisiana do not, by default, require something in writing.
"A contract is an agreement between 2+ persons by which obligations are created, modified or extinguished.
Four elements of a valid contract: capacity, consent, lawful cause, and lawful object."
The answer given here should be considered general in nature and should not be considered legal advice, or that there has been an attorney-client relationship established. It is always suggested to retain the counsel of a lawyer for legal advice specific to your situation.