The purpose of a service mesh in microservices architecture is to manage the interactions between services in a distributed system. It functions as an infrastructure layer that provides crucial capabilities such as service discovery, load balancing, and communication security. By abstracting these functionalities, service meshes allow developers to focus on business logic without worrying about network complexities. They enable features like traffic management, observability, and fault tolerance, ensuring reliable service interactions. Additionally, service meshes facilitate centralized policy enforcement for security and compliance, thereby optimizing overall application performance and resilience in cloud-native environments.