If you got access to a credit card, then it is good for you .
but many gamers or youngester do not have credit card . Their only fund to pay for games is their monthly allowance
So they need to use cash to somehow get the credit. Therefore this gift card is one method.

also last time PSN have network security issue.  If anything happened, your credit card details might be stolen .

also prepaid credit , will allow you to control your expenses