Implementing redundant routers in a network design provides a backup in case one of the routers fails, thus allowing for continuous network operation and minimizing downtime. While redundant firewalls and redundant switches also provide high availability, redundant routers ensure that if one pathway is unavailable, another can immediately take over the routing responsibilities, which is especially crucial for maintaining external connectivity. NIC teaming does provide fault tolerance and increased bandwidth but does not offer the same level of routing redundancy as having multiple routers.