I don't know why others make the distinction. To me, it's part ethics, part strategy. The strategy part is understanding the difference between a fight (stay in until you win or lose) and defense (if you have a chance to escape, generally, you should take it). The ethics part is more personal, but it ties to the idea of avoiding unnecessary risk, too (a principle of self-protection). If someone offers violence, it's generally better to decline than to engage, if that's an option. So, if someone loses their mind in anger (the "emotional hijacking" I referenced in another thread) and wants me to go out back with them, I won't go. I simply won't agree to the fight. Of course, we could probably come up with a circumstance where agreeing to the fight might be acceptable, but it wouldn't normally be so.
If we don't make a distinction between what is self-defense and what isn't, we put all fighting on the same moral plane. I think most people would agree that there's nothing wrong with fighting to defend oneself from an attack, and that any injuries you receive while doing so are the "fault" of the attacker. I think most people would also agree that agreeing to fight someone who just wants to hurt you because they are mad at you is probably a bad idea, and that any injuries you sustain in that situation are at least partly your own responsibility.