Maybe it’s not for the public.
Maybe it’s to force the reporters who are covering the campaign to become aware of the Clintons’ past behavior.
Also, perhaps to get into Hillary’s head a little? Trump likes to be unpredictable. Trump bringing this up has got to make Hillary’s heart skip a beat. “What else is he going to bring up?”, she’s probably thinking.
It’s got to be demoralizing to the Clintons to realize there’s no safe haven for them. Everything’s on the table. Keeping your enemy off guard is a good thing.