In business, agreements matters a whole lot, most good work ethics depends on it, a default in any form of agreement is bad for business and investment. A client, any customer who breaches agreement will cost you a fortune in any form of deal if not put you into trouble.
Verbal agreement doesn't count, people don't hold their words in esteem hence a carefully written and properly documented forms of affirmation is needed so that in the event of default, a legal case could be filed. Unfortunately, written and signed documents too are not duly respected anymore, i know this for a fact based on my experience in the last couple of days with a client.
Sadly, it doesn't matter if you keep your own end of the bargain, defaulters will still default based on the lack of good business intelligence in a vast majority of people and organization. Most companies are also guilty of these unprofessional act as well as individuals in business. Its a real headache if you are going into any form of business and you plan to keep it running, be advised to always have a written and signed agreement on paper, its the best evidence against institutional theft and breach of contracts.