Say for example, my machine is multi-homed and has two network interfaces:
I create two UDP sockets one listening on (1) and other on (2). The i am assuming the interface is already assigned, then why do i need IP_MULTICAST_IF and IPV6_MULTICAST_IF?
The IP_MULTICAST_IF or IPV6_MULTICAST_IF settings tell your socket which interface to send its multicast packets on. It's a separate, independent setting from the interface that you bound your socket to with bind(), since bind() controls which interface(s) the socket receives multicast packets from.
(Granted, the BSD sockets API implementers could have made the assumption that a socket would always want to send packets out over the same interface it receives packets on, but that would make a number of use cases more difficult; for example, if you are using a single socket to receive multicast packets from all interfaces (via INADDR_ANY), then when sending a packet using that socket you'd still need a way to specify which multicast interface you want that packet to be sent over)