The primary function of a router is to direct data between computer networks. It connects multiple devices within a local network and manages traffic by forwarding data packets to their intended destinations. Routers use IP addresses to determine the best paths for data transit across interconnected networks. They play a crucial role in maintaining network security by incorporating firewalls and other protective measures. Additionally, rouuters often provide functionalities like network address translation (NAT) and dynamic host configuration protocol (DHCP), facilitating communication between devices on a local network and the internet.