If I had a choice I would go back to the old ways of a handshake and keeping your promises. These days unfortunately people can’t or won’t stand by their commitments. Also, people’s interpretation of what was agreed differs anyway so there has to be a way of recording the agreement.
Essentially, a contract confirms the scope of works, the cost and programme, period. Payment terms and how the works are to be carried out, safety provisions etc. are also set out in the contract. There are statutory requirements like CDM, The Construction (Design and Management) Regulations and Health and Safety Regulations and the contract will help determine the requirements that apply in a particular project. A contract also focuses the mind on the completion date.
A contract sets out what has been agreed which is for the benefit of both parties.
Without a written contract a client is in the hands of the contractor and his terms and conditions. Unless these terms cover such an eventuality, there will be no mechanism to deal with things going wrong and no incentives for the contractor to keep to the initial agreement like completing on time. If the agreement is based upon an estimate, even the cost is not agreed!
I always recommend using a standard form of contract like one of the suite of JCT contracts currently available.