Debit Card and Credit Card both are plastic money. but they are different altogether.
Debit card means you can use this card up to balance in your respective bank account connected to debit card. Example: if u have balance in bank account Rs 10, 000 than u can use it up to Rs 9, 000(it is assumed that minimum balance require is Rs 1000)
on the other hand you can use your card upto limit of credit card. than you have to pay the same on due date. Example: if u have credit card limit of Rs 10000 than u can use your credit card upto Rs 10000/-
Credit card is very useful when you have no cash balance and you have urgent require of money.
Credit Card is provides cheap money and with rewwards on using card.