A parenthetical often provides clarification or extra details without changing the main message. His achievements (which were many) were not what defined him.
Pro Tip 2/3
Subtle Emphasis
Use a parenthetical to subtly emphasize a point or opinion within a sentence. She was, in my opinion (and I'm not alone), the best candidate.
Pro Tip 3/3
Audience Connection
Parentheticals can make your writing feel conversational, as if speaking directly to the reader. The venue (you've been there, right?) was packed.