A written contract is the best way for any given business AND for a client/customer to ensure the agreement protects both parties. A contract must be obtained with full disclosure and it must be fair (it cannot be obtained by deceit, fraud, and/or misrepresentation).
A written contract is essential to protect both parties.
In Washington, oral contracts are also enforceable (with a few exceptions). An oral agreement which contains an offer, an acceptance, and legal consideration paid by both parties can be fully enforceable and award damages against the party who breached the agreement.
The main issue with an oral contract is that often, it is very hard (if possible at all) to prove the terms, duties, and considerations of the contract. Although the parties can attempt to prove the important terms of the agreement, it is up to the party enforcing the contract to show, by the preponderance of the evidence, that the term was entered into and breached by the other party.