The purpose of a service mesh in microservices architectures is to provide a dedicated infrastructure layer that manages communication and connectivity between services. It handles concerns such as service discovery, load balancing, security, and monitoring, allowing developers to focus on building business logic instead of worrying about network complexity. A service mesh enhances security by enforcing policies for communication and access control between microservices. Its observability features help teams monitor performance metrics and troubleshoot issues effectively, thus improving the overall reliability of distributed applications.