Using your credit card is like getting a loan. When you use your credit card to buy something, you are borrowing money.
Some people use a credit card to buy things they cannot afford right now. Some people use a credit card to help build or improve their credit history. Sometimes it is just easier not to carry cash. Sometimes it is easier to pay once a month for the things you buy.
You pay less for your credit if you pay everything you owe every month.