It is a known flaw of the human mind that easy payment methods, like credit cards and electronic debits, don't register as painful financial transactions as strongly as counting out and handing over those funny bits of green paper. Because of this using a credit card, without resort to some involved mental gymnastics, almost always means you spend more than you might if you used cash.

http://www.wired.com/wiredscience/2011/03/the-high-cost-of-easy-payments/

Keeping the spending of money both inconvenient and uncomfortable saves you money. Saving just a few dollars a day can mean the difference between staying square and afloat, and being at he tender mercies of banks and bill collectors.