You use this to explain your actions, especially if they were unpopular, unkind or morally dubious. The idea is if you were defending yourself in court. "I realise that wasn't nice of me to say that about her, but in my defence she had always treated me far worse." "Yes, I kept your favourite book. In my defence, I had no way of contacting you to return it."

In my defense, I have to admit that I forgot to call you last night because my mother came to see me. I am sad that I lied to you. In my defense, all I can say is that I am very sorry and it will never happen again.