1. Sue you in court. If you have public record, this will hurt you in your credti score.
2. Hire collection agents or sell the debts to collectors. If you pull your credit report, there is an item for all colleciton records, which list to whom you owe the debt and how much is the amount.
In credit score model, both public record and collection record are negative factors. If they are most important factors that make your credit score lower, credit bureau has to disclose them as "adverase action".