The "defending" call serves several purposes.
First off, it lets the rest of the flight know that someone is under a threat that is significant enough that they are abandoning whatever else it is that they're doing and just concentrating on surviving that particular attack and defending against that threat.
In addition it has to do with deconfliction responsibilities between flight-mates. The normal contract is that the wingmens' job is always to deconflict with their flight lead. When someone is under attack, it is difficult (if not impossible) to simultaneously defend against a threat (weather that be a SAM or another aircraft) and also maintain deconfliction from other aircraft in the formation.
So, the "defending" call is essentially saying, "hey, other members of the flight, get outta my way because I'm not looking at you, I'm defeating something that is about to kill me."
There are some additional tactical meanings, but that's the gist of it.
