Gifting Apps with Credit
I don't choose to have a cc linked to my Apple ID and instead purchase gift cards. Can I send an app as a gift using credit? If not, why?
iPad, iOS 7.0.4
You can't use your balance for gifitng, that can only be used to buy items for yourself. If you want to gift items or an amount then you will need to have a credit card on your account - from http://support.apple.com/kb/HT2736 :
Note: When purchasing an iTunes Gift, a valid credit card must be on file; you can't purchase an iTunes Gift with store credit. You can't redeem purchased gifts outside of the country of purchase.
As far as I know Apple haven't said why it's like that, but I think that it's fairly common practice.