Contract-First Development is about making your intent clear across team members and external teams. By using a tool like OpenAPI, we can define our programming API clearly at the outset. We also update the "contract" before we implement changes to the code. Further benefits include:
So that everyone can be productive in parallel AND we can have strong guarantees of the resulting applications working together properly. Instead of UI development being blocked awaiting a working service to code against, UI and Service developers can work in parallel. Also, if you define the contract first when developing or changing API interfaces, you have a clearer "Definition Of Done" and "Acceptance Criteria" for the work.
Check out these great links which can help you dive a little deeper into running the Contract-First Development practice with your team, customers or stakeholders.