It's my understanding that credit repair companies will contest many of the debts that show on your report with your creditors. The creditors are asked to provide proof that you entered into an agreement (the contract with signatures etc.) on the expectation that some will have lost it especially if the debt is dated.

The first time I ever checked my credit report, I was 22, and I learned immediately that my credit was bad. The reason being was that I had apparently been the victim of identity theft where someone had opened an Amex card in my name, maxed it out, and defaulted. Amex never ever contacted me about it, nor had any debt collector. I called Experian and Equifax about it and they helped put me in touch with Amex. I told them I never opened a card and asked for a copy of the signed agreement. They never sent it and the debt was promptly removed from my credit reports. No follow up was required.

This was a case where a debt actually wasn't mine, but it was still an exercise in the "prove the debt" is mine scenario in which the debt was removed and my credit repaired. I think that's a tactic credit repair companies use.