The main advantages of VIPs over the physical IPs are the nearly limitless mobility, simplicity, source address choice for outbound connections, support for host mask, and high availability. Moreover, a virtual IP address excludes a host's reliance upon single network interfaces and provides a failover alternative to individual machines. Previously, if an interface failed, all its connections were lost. Now incoming data batches are directed to the system's VIP address; then, they're distributed by dynamic routing protocols through the actual network interfaces.
For this to work, the host needs an interior gateway protocol - the Open Shortest Path First (OSPF), for instance - and it has to become a router for the rest of the network. When one web interface fails, the OSPF guides traffic through another interface. VIPs have multiple implementation set-ups and variations, including Proxy Address Resolution Protocol (Proxy ARP) and Common Address Redundancy Protocol (CARP).