Even if you return all your books on time, it's still valuable, because of the huge jump they're seeing in the rate at which books are returned.
Random anecdote: About 15 years ago, I was looking for a somewhat obscure book, and found that it was something like 6 months overdue. Later on, while hanging out with a friend, I mentioned this to a friend who had a common interest in the subject matter, with some comment to the effect of, "What happened there!?"
Well, this friend went over to his bookshelf, picked a book off the shelf, and handed it to me. It was the very same 6 month overdue book I had been looking for. He explained that he had lost it somewhere, and, by the time he had found it, the fines were already up to something like $50, which he couldn't afford, and he wasn't done reading it yet, so he figured he might as well keep it since it was the last library book he'd ever be checking out.
IIRC the statistic quoted when CPL announced this program suggests that some version of that story was playing out for thousands and thousands of books every year. Which negatively affects access to information for everyone in the city, not just the people who were getting slapped with fines.