Oral agreements are difficult, expensive, and sometimes impossible to enforce, so you should assume that it is always better to have an agreement in writing. Any agreement that is not in writing is vulnerable to the three “M’s:” memory, misinterpretation, and mood.
Memory is never perfect. It’s completely plausible that two-honest, well-meaning people will have two completely different memories about an oral agreement.
Misinterpretation occurs when two parties agree to something, but what they think they’re agreeing to isn’t the same. Putting the agreement in writing may show that the parties did not accurately understand each other in prior discussions.
Mood changes how a person feels about an agreement. If someone discovers that a previous agreement doesn’t feel right a few months or years later, they may change their mind and attempt to change the terms of the contract.