Because every time you use your credit card to withdraw case, more fees kick in:

• Cash advances can carry an upfront fee of 2 percent to 4 percent of the amount advanced.
• The advances have a higher interest rate than regular card charges.
• Interest charges begin to mount as soon as the money comes out of the ATM.
• Many issuers also require you to pay down the balances for purchases before you pay down the higher-interest cash advance balance.

Here's an example of how these fees kick in:

Assume you bought a television for $500 on your card and then took out $50 in cash. Even though you pay the $50 back the next day, you still lose your interest-free period because the credit provider deems you pay the cash back last.

As a result you will still owe the $50, but you will now only owe $450 on the $500 worth of purchases.

You'll continue to forfeit your interest-free period up until you have completely paid back the full $550. Any future purchases will still be ahead of the $50 in the payback line.