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.
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).