Your credit score partly depends on your credit utilization – the amount of debt you carry as compared to the total amount of debt available to you. If all of your credit cards are maxed out, opening a new one increases your available debt and causes your utilization ratio to go down, and that could help your score. But your score will take a ding any time you carry a high balance on any one card. So if you transfer multiple balances to a single card and get close to (or reach) your credit limit, your score will suffer even if your other cards are paid off.
With poor credit, you may not be able to get approved for new credit products like credit cards. Although you may still be able to take out an auto loan or a mortgage, you’ll pay a much higher interest rate because of your low credit score. Compared to a borrower with good credit, someone with poor credit can pay $50,000 more in interest on a mortgage. Over an entire lifetime, you could end up paying over $200,000 more in unnecessary interest just because of bad credit.