Hello Rupesh,
usage of address families is not limited to IPv4 and ipv6:
Multi protocol BGP allows for:
vpnv4 AF that provides signalling plane for MPLS L3 VPN
ipv4 vrf vrf-name : context for VRF related sessions
ipv4 multicast: to be used in inter-domain multicast to provide a way to satisfy RPF check for sources coming from other BGP AS.
other address families are out there for inter-AS EoMPLS for example.
Using ipv4 address-family directly also for ipv4 unicast only contexts makes more consistent the aspect of the configuration.
Hope to help
Giuseppe
