The primary function of performance testing in software development is to ensure that applications perform optimally under varying loads and conditions. It assesses aspects such as response time, scalability, and reliability to identify any bottlenecks or weaknesses within the system. Performance testing helps developers understand how their application behaves under different scenarios, enabling them to make necessary adjustments before launch. This proactive measure not only enhances user experience but also helps prevent costly downtime and customer dissatisfaction following deployment.