The primary purpose of a subnet in computer networking is to divide a larger network into smaller, manageable segments. Subnetting enhances network performance and security by organizing devices in groups, making it easier to control traffic and apply security policies. Each subnet operates with its unique subnet mask, enabling better distribution of IP addresses and reducing broadcast domains. This segmentation helps optimize bandwidth usage and simplifies network management, especially in larger environments where multiple users and devices necessitate streamlined communication and resource allocation.