Many places have readily adopted credit cards and cashless payments, and a credit card can reduce how much cash you need to carry in case of theft. But there are countries whose businesses still conduct most of their transactions in cash. Travel around parts of Europe and you may find many restaurants and small businesses require a minimum amount (usually the equivalent of $10 or more) before they'll accept a credit card. That's why it's a good idea to have a debit card on hand as well.
Remember that exchanging money yourself is usually far more expensive and time-consuming than withdrawing the funds using a debit card. Many currency exchanges charge fees to exchange your dollars and give you a less favorable rate. When you use your debit card to withdraw the cash at a foreign ATM, you'll get the bank's wholesale rate -- as long as you decline any currency conversions the ATM offers to make for you.
However, there may be international ATM fees you need to keep an eye on. Before you travel, ask your bank about what fees you'll be charged on withdrawals.