Here’s basic reason for credit card - rewards.
If you had HDFC Infinia -
If a product costs 1 lakh, you will pay 1 lakh and get upto 30% discount in form of reward point (extreme example from Infinia card).
Say you made many such payments over few months. Accumulated reward points can then be used for hotel/flight booking etc when you go on tour. Depending on terms and conditions, it will be up to 70% cheaper because you had reward points earned by previous spend.
Keep in mind, I used Infinia as example, lower/other cards have different use case/points structure.
So basically, over the year if you spend 10 lakhs and you optimised for reward points, you’ll have 3 lakh reward points. Which means a good trip to Singapore for family of 2 is now sponsored by hdfc for using their card.