The primary use of an API (Application Programming Interface) in software development is to enable communication between different software applications. APIs define a set of rules and protocols that allow one software component to interact with another, facilitating data exchange and functionality sharing. This promotes interoperability and integration of services, enhancing application performance. APIs also simplify the development process by providing predefined functions and allowing developers to leverage existing technologies. Overall, they play a crucial role in building connected and efficient software systems.