You, hypothetically, are a bit of a rough person in your early 20s. You do some Crimes, one of which was against me. We'll say it was enough to give you a hefty fine and a year or less of jail time.
I don't press charges.
20 years pass. You grow up. You've changed your ways. You've become a squeaky-clean individual. You've put all that behind you. You become a healthy member of society. Your career's underway, you live in your own place, you may or may not have started a family.
Hey, remember that Crime I didn't press charges about at the time? Well, surprise, motherfucka. I've been waiting for this moment to do so. To the courts you go, get your ass fined, thrown in jail, and give you a criminal record, all so it'd hurt you that much worse now that you have your roots planted in your life.
Actually, you did Crimes to several people, right? Let's get them all in on this action! We'll just kind of trickle the suits in, one-by-one. Let one resolve, give it a few months, the next guy presses charges about his. Just kind of a steady flow of skeletons in the closet that you have to either defend against (and how are you gonna do that? It's 20 years old, hope you have evidence for your side somewhere in the attic) or take the sentencing of (which will do wonders for that career of yours), just to make your life hell.