Hiring an API level testing developer, particularly for REST, is crucial for ensuring the seamless integration of your systems. They ensure that your APIs are functioning as expected, are reliable, secure, and can efficiently handle expected loads. The developer can identify and address any issues early in the development cycle, thus saving time, reducing costs, and improving the quality of your software. Their expertise can greatly enhance the user experience, as they ensure that data is correctly exchanged between different software applications. Ultimately, they play a pivotal role in your product's success.