Contracts
Contracts are important as they protect both you and the person or business that has hired you. A contract is helpful because it defines the type of work you will provide and sets expectations for the work to be delivered. It also provides documented evidence of the agreement that you will be working as an independent contractor and not as an employee (which has different tax implications).
A contract allows both you and the hirer (your client) to think through key aspects of the working relationship, and makes clear additional provisions of the working relationship, such as keeping parts of the work confidential, making clear who owns the work product, how the contractor can talk about the project in their marketing, and many more arrangements.