Well, if you don't have a contract how do you define what the deliverables are? Also, remuneration rates, allowable expenses and all that.......... lawyers can be very expensive

A system that is not properly documented is difficult if not impossible to support.........also to update. Are you expected to support it after it goes live?

Supposing this guy gets killed in a car crash or whatever (I will give you an alibi MLF) what then? You will fetch up paying a fortune for someone to analyse the system to find out what it does and how it works?

Otherwise you are faced with a complete re-write?

The basic reasons for contracts and documentation are to avoid downstream costs. They are pre-emptive rather than pedantic.