This is probably obvious to most people doing business, but there are still a large number of people that still believe in doing business by a shake of the hand. The truth of the matter is that with important transactions, your agreement should be in writing even amongst good friends.

A written agreement is a protection for both parties.  A well-drafted agreement will anticipate not only remedies for bad actors in a transaction but also provide solutions for results that are unintended.  Just as it is important to provide accurate expectations for your customers, it is important to create proper expectations with those who you do business with on a B2B level.

If you peruse the small claims docket in your local area you will find many of the breach of contract actions have to do with oral agreements; this is not a coincident.  Uncertainty always leads to litigation.  Your objective when entering into a business transaction with another party is to prevent or at least minimize the chance of entering into a dispute which can not be resolved out-of-court.  Litigation is expensive and risky for both parties.  A party who breaches well-drafted written contract that is clear will not put up as much of a fight compared to a party who breaches a contract which is oral and ambiguous.