To sell a home, there is a contract called a Listing Agreement. It is necessary.
To buy a home, there is a buyers contact with the buyers agent, that is optional. Like John, I feel a trust between buyer and agent is more important than a piece of paper. However, to purchase a home you will need to sign a purchase contract / offer. This will have all the terms and price which you are offering.
To re-cap; You do not need to sign a contract with an agent to buy a home.
Sit down with an agent / Broker and discuss your wants, needs, and concerns - and don't sign anything you do not fully understand.
Many Realtors will represent buyers without a signed contract. I allow my performance to bind my clients to me. If my clients are not satisfied with my performance a contract only increases their displeasure.
It is unlikely that any Realtor will represent a seller without a signed listing contract.