lots of people dont NEED them but its easier to use them over a check (hell some places wont take a personal check) and safer then paying cash or using a debit card
and they also have perks like airline miles, cash back and extended warranty - ive never carried a balance and i use mine for the reasons stated above
ordering things online with a debit card is a bad idea, CC is the only other option
Problem is if you need to rent a car or hotel room you have to have a CC or let them put a $500 hold on a debit card (maybe more/less but $500 is what some use).

