The main function of a router in a computer network is to direct data packets between devices and networks, ensuring they reach their intended destinations. Routers connect multiple networks, such as a local area network (LAN) to the Internet, and manage traffic by determining the optimal path for data transmission. They also provide functionalities such as firewall protection and network address translation (NAT). By efficiently routing data, routers play a critical role in maintaining the performance and security of computer networks.