Link monitoring helps the firewall to failover if a physical link or group of links fail. This check is necessary to make sure traffic continuity to the firewall. If the link/s fail then firewall cannot process and forward traffic and hence it fails over to the other peer to receive the traffic. Similary in Path monitoring the firewall monitors a specified destination IP address to be reachable through pings indicating the connection is up for HA to function. if the pings fail then the path to the destination IP is considered fail and hence it will failover the firewall to make sure the path is connected for HA to function at optimal levels. It is recommended to have Link/Path Monitoring enabled to have traffic continuity through the firewalls.