Because it automatically stores-away money for each purchase. Some users enjoy the piece of mind of not having a bill at the end of month, and this provides the funds in order for that to happen. For me, I personally hate budgeting with credit cards and hate seeing my statement at the end of the month cause my checking balance is not accurate then. I would use a debit card more, but I get little protection and no benefits. This app may not be for you and that's okay!