Interest rates have a big impact on the cost you pay for borrowing money. Loan payments are made up of interest and principal (what you borrow). A low-interest rate loan is easier to repay because there’s less interest added to your monthly payment. Lower interest rates are highly sought after because you pay less money to the bank who's loaned you money.
People with a lower credit score typically pay higher interest rates when borrowing money which increases the cost of borrowing.