Partners are not required to enter into a partnership agreement. The law (primarily written in the late 19th Century) will impose a default set of rights and obligations to govern the partnership and the relationship between the partners if they don't.
Partnership agreements are voluntary, they are designed to provide clarity and certainty to the relationship between the partners, and protect the partners’ investment in the business (over and above the default provisions set-out in general partnership law).
It is recommended that all partnerships enter into a partnership agreement. Having a clear agreement in place lets the partners know where they stand in relation to each other and the partnership business.
Many costly disputes have been resolved and problems avoided as a direct result of entering into a partnership agreement.