Here’s a good example of when a reputable credit repair service can help you do something you may not be able to accomplish yourself. If you have a collection account that’s been sold to a few different debt collectors, it can appear on your credit report multiple times. That information is accurate but having that one debt dinging your credit score multiple times doesn't meet the “fair” standard that Padawer mentioned.
Some of your creditors and lenders might report only to one of the credit bureaus. And, since credit bureaus don’t typically share information, it’s possible to have different information on each of your reports. Ordering all three reports will give you a complete view of your credit history and let you repair your credit at all three bureaus instead of just one.
Credit scoring models usually take into account how much you owe compared to how much credit you have available, called your credit utilization rate or your balance-to-limit ratio. Basically it's the sum of all of your revolving debt (such as your credit card balances) divided by the total credit that is available to you (or the total of all your credit limits).