„Boxer Joe Foster had fought over 100 opponents when he retired in 1994.” Why someone used Past Perfect in this senence?
The answer is simple and was given in the comment. Fighting 100 opponents came before the 1994 retirement (we are in 2019 now). He had done that before he retired. That’s what the past perfect means and how it‘s used. It’s just basic grammar in most all languages I know of.